i386/sev: Crash in pc_system_parse_ovmf_flash caused by bad firmware file
Description of problem:
A specially-crafted flash file can cause the `memcpy()` call in
`pc_system_parse_ovmf_flash` (`hw/i386/pc_sysfw_ovmf.c`) to READ out-of-bounds
memory, because there's no check on the `tot_len` field which is read
from the flash file.  In such case, `ptr - tot_len` will point to a
memory location *below* `flash_ptr` (hence the out-of-bounds read).

This path is only taken when SEV is enabled (which requires 
KVM and x86_64).
Steps to reproduce:
1. Create `bad_ovmf.fd` using the following python script:
   ```
   from uuid import UUID
   OVMF_TABLE_FOOTER_GUID = "96b582de-1fb2-45f7-baea-a366c55a082d"
   b = bytearray(4096)
   b[4046:4048] = b'\xff\xff'   # tot_len field
   b[4048:4064] = UUID("{" + OVMF_TABLE_FOOTER_GUID + "}").bytes_le
   with open("bad_ovmf.fd", "wb") as f:
       f.write(b)
   ```
2. Build QEMU with `--enable-sanitizers`
3. Start QEMU with SEV and the bad flash file:
   ```
   qemu-system-x86_64 -enable-kvm -cpu host -machine q35 \
                      -drive if=pflash,format=raw,unit=0,file=bad_ovmf.fd,readonly=on \
                      -machine confidential-guest-support=sev0 \
                      -object sev-guest,id=sev0,cbitpos=47,reduced-phys-bits=1,policy=0x0
   ```
4. QEMU crashes with: `SUMMARY: AddressSanitizer: stack-buffer-underflow`
Additional information:
Crash example:

```
$ sudo build/qemu-system-x86_64 -enable-kvm -cpu host -machine q35 \
                                -drive if=pflash,format=raw,unit=0,file=bad_ovmf.fd,readonly=on \
                                -machine confidential-guest-support=sev0 \
                                -object sev-guest,id=sev0,cbitpos=47,reduced-phys-bits=1,policy=0x0
==523314==WARNING: ASan doesn't fully support makecontext/swapcontext functions and may produce false positives in some cases!
=================================================================
==523314==ERROR: AddressSanitizer: stack-buffer-underflow on address 0x7f05305fb180 at pc 0x7f0548d89480 bp 0x7ffed44a1980 sp 0x7ffed44a1128
READ of size 65517 at 0x7f05305fb180 thread T0
    #0 0x7f0548d8947f  (/usr/lib/x86_64-linux-gnu/libasan.so.5+0x9b47f)
    #1 0x556127c3331e in memcpy /usr/include/x86_64-linux-gnu/bits/string_fortified.h:34
    #2 0x556127c3331e in pc_system_parse_ovmf_flash ../hw/i386/pc_sysfw_ovmf.c:82
    #3 0x556127c21a0c in pc_system_flash_map ../hw/i386/pc_sysfw.c:203
    #4 0x556127c21a0c in pc_system_firmware_init ../hw/i386/pc_sysfw.c:258
    #5 0x556127c1ddd9 in pc_memory_init ../hw/i386/pc.c:902
    #6 0x556127bdc387 in pc_q35_init ../hw/i386/pc_q35.c:207
    #7 0x5561273bfdd6 in machine_run_board_init ../hw/core/machine.c:1181
    #8 0x556127f77de1 in qemu_init_board ../softmmu/vl.c:2652
    #9 0x556127f77de1 in qmp_x_exit_preconfig ../softmmu/vl.c:2740
    #10 0x556127f7f24d in qemu_init ../softmmu/vl.c:3775
    #11 0x556126f947ac in main ../softmmu/main.c:49
    #12 0x7f05470e80b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
    #13 0x556126fa639d in _start (/home/dmurik/git/qemu/build/qemu-system-x86_64+0x2a5739d)

Address 0x7f05305fb180 is located in stack of thread T3 at offset 0 in frame
    #0 0x556128a96f1f in qemu_sem_timedwait ../util/qemu-thread-posix.c:293


  This frame has 1 object(s):
    [32, 48) 'ts' (line 295) <== Memory access at offset 0 partially underflows this variable
HINT: this may be a false positive if your program uses some custom stack unwind mechanism, swapcontext or vfork
      (longjmp and C++ exceptions *are* supported)
Thread T3 created by T0 here:
    #0 0x7f0548d28805 in pthread_create (/usr/lib/x86_64-linux-gnu/libasan.so.5+0x3a805)
    #1 0x556128a97ecf in qemu_thread_create ../util/qemu-thread-posix.c:596

SUMMARY: AddressSanitizer: stack-buffer-underflow (/usr/lib/x86_64-linux-gnu/libasan.so.5+0x9b47f)
```