Single stepping Windows 10 bootloader results in Assertion `ret < cpu->num_ases && ret >= 0' failed.
Description of problem:
When I am trying to debug Windows bootloader, I see an assertion error in QEMU when single stepping some instructions in SeaBIOS.

```
qemu-system-i386: ../hw/core/cpu-sysemu.c:77: cpu_asidx_from_attrs: Assertion `ret < cpu->num_ases && ret >= 0' failed.                                        
```
Steps to reproduce:
1. Download / construct `w.img`, see above
2. Start QEMU using `./qemu-system-i386 --drive media=disk,file=w.img,format=raw,index=1 -s -S -enable-kvm`
3. Start GDB using `gdb --ex 'target remote :::1234' --ex 'hb *0x7c00' --ex c --ex 'si 1000' --ex q`
4. See error message
Additional information:
The GDB script first breaks at 0x7c00, then tries to execute 1000 instructions using single step (`si`). On my machine, after executing around 772 instructions, the assertion error in QEMU happens. 
Here is an interactive GDB session on my machine. 

```
(gdb) hb *0x7c00
Hardware assisted breakpoint 1 at 0x7c00
(gdb) c
Continuing.

Breakpoint 1, 0x00007c00 in ?? ()
(gdb) d
Delete all breakpoints? (y or n) y
(gdb) si 770
0x000f7d7b in ?? ()
(gdb) x/10i $eip
=> 0xf7d7b:	mov    $0x7d85,%ebx
   0xf7d80:	out    %al,$0xb2
   0xf7d82:	pause  
   0xf7d84:	hlt    
   0xf7d85:	mov    %bp,%sp
   0xf7d88:	jmp    0xf7dd1
   0xf7d8a:	mov    %cx,%si
   0xf7d8d:	mov    $0x1,%ax
   0xf7d91:	add    %al,(%eax)
   0xf7d93:	callw  0x6b66
(gdb) si
0x000f7d80 in ?? ()
(gdb) info reg
eax            0xb5                181
ecx            0x5678              22136
edx            0x0                 0
ebx            0x7d85              32133
esp            0xe96d4             0xe96d4
ebp            0xfed4              0xfed4
esi            0xe0346             918342
edi            0xefd91             982417
eip            0xf7d80             0xf7d80
eflags         0x6                 [ IOPL=0 PF ]
cs             0x8                 8
ss             0x10                16
ds             0x10                16
es             0x10                16
fs             0x10                16
gs             0x10                16
fs_base        0x0                 0
gs_base        0x0                 0
k_gs_base      0x0                 0
cr0            0x11                [ ET PE ]
cr2            0x0                 0
cr3            0x0                 [ PDBR=0 PCID=0 ]
cr4            0x0                 [ ]
cr8            0x0                 0
efer           0x0                 [ ]
...
mxcsr          0x1f80              [ IM DM ZM OM UM PM ]
(gdb) si
0x000f7d82 in ?? ()
(gdb) info reg
eax            0xb5                181
ecx            0x5678              22136
edx            0x0                 0
ebx            0x7d85              32133
esp            0xe96d4             0xe96d4
ebp            0xfed4              0xfed4
esi            0xe0346             918342
edi            0xefd91             982417
eip            0xf7d82             0xf7d82
eflags         0x6                 [ IOPL=0 PF ]
cs             0x8                 8
ss             0x10                16
ds             0x10                16
es             0x10                16
fs             0x10                16
gs             0x10                16
fs_base        0x0                 0
gs_base        0x0                 0
k_gs_base      0x0                 0
cr0            0x11                [ ET PE ]
cr2            0x0                 0
cr3            0x0                 [ PDBR=0 PCID=0 ]
cr4            0x0                 [ ]
cr8            0x0                 0
efer           0x0                 [ ]
...
mxcsr          0x1f80              [ IM DM ZM OM UM PM ]
(gdb) si
Remote connection closed
(gdb) 
```

This bug was first incorrectly filed in KVM's bug tracker at <https://bugzilla.kernel.org/show_bug.cgi?id=216003>.