virtio: bounce.in_use==true in virtqueue_map_desc()
Description of problem:

Steps to reproduce:
1. Build EDK II (edk2-stable202311) for riscv64
2. Build UEFI SCT (commit 81dfa8d53d4290) for riscv64
3. Run the UEFI SCT
4. Observe the message "qemu: virtio: bogus descriptor or out of resources" after which the execution stalls.

The full procedure is described in https://github.com/xypron/sct_release_test

To save time you can call `sct -u` and select only test 'MediaAccessTest\\BlockIOProtocolTest'. Run it with `F9`.
Additional information:
virtqueue_map_desc() may be called for a large buffers size `sz`. It will then call dma_memory_map() multiple times in a loop. In address_space_map() `bounce.in_use` is set to `true` on the first call. Each subsequent call is bound to fail.

To verify this is the cause I applied the following diff:

```plaintext
diff --git a/system/physmem.c b/system/physmem.c
index a63853a7bc..12b3c2f828 100644
--- a/system/physmem.c
+++ b/system/physmem.c
@@ -3151,12 +3151,16 @@ void *address_space_map(AddressSpace *as,
 
     if (!memory_access_is_direct(mr, is_write)) {
         if (qatomic_xchg(&bounce.in_use, true)) {
+           fprintf(stderr, "bounce.in_use in address_space_map\n");
+
             *plen = 0;
             return NULL;
         }
         /* Avoid unbounded allocations */
         l = MIN(l, TARGET_PAGE_SIZE);
         bounce.buffer = qemu_memalign(TARGET_PAGE_SIZE, l);
+       if (!bounce.buffer)
+           fprintf(stderr, "Out of memory in address_space_map\n");
         bounce.addr = addr;
         bounce.len = l;
```

and saw this output:

```plaintext
Logfile: "\sct\Log\MediaAccessTest\BlockIOProtocolTest0\ReadBlocks_Conf_0_0_8261
59D3-04A5-4CCE-8431-344707A8B57A.log"
Test Started: 12/02/23  08:43a
------------------------------------------------------------
Current Device: Acpi(PNP0A03,0)/Pci(3|0)
Bounce.in_use in address_space_map
qemu: virtio: bogus descriptor or out of resources
```

See related bug #850.