virtio-balloon: Assertion `mrs.mr' failed.

Hello,

Reporducer:

cat << EOF | ./qemu-system-i386 -display none -machine accel=qtest, -m \
512M -machine q35 -nodefaults -device virtio-balloon -qtest stdio
outl 0xcf8 0x80000890
outl 0xcfc 0x2
outl 0xcf8 0x80000891
inl 0xcfc
EOF

Stack-trace:

#0 0x7d5ca4f0a00b in raise /build/glibc-LcI20x/glibc-2.31/sysdeps/unix/sysv/linux/raise.c:51:1
#1 0x7d5ca4ee9858 in abort /build/glibc-LcI20x/glibc-2.31/stdlib/abort.c:79:7
#2 0x7d5ca4ee9728 in __assert_fail_base /build/glibc-LcI20x/glibc-2.31/assert/assert.c:92:3
#3 0x7d5ca4efafd5 in __assert_fail /build/glibc-LcI20x/glibc-2.31/assert/assert.c:101:3
#4 0x5c5e521b37db in virtio_address_space_lookup /src/qemu/hw/virtio/virtio-pci.c:620:13
#5 0x5c5e521b37db in virtio_address_space_read /src/qemu/hw/virtio/virtio-pci.c:692:10
#6 0x5c5e521b37db in virtio_read_config /src/qemu/hw/virtio/virtio-pci.c:814:13
#7 0x5c5e51fa3d45 in pci_host_config_read_common /src/qemu/hw/pci/pci_host.c:118:11
#8 0x5c5e51fa465b in pci_data_read /src/qemu/hw/pci/pci_host.c:154:12
#9 0x5c5e51fa4ac5 in pci_host_data_read /src/qemu/hw/pci/pci_host.c:199:12
#10 0x5c5e52766f21 in memory_region_read_accessor /src/qemu/system/memory.c:445:11
#11 0x5c5e52753860 in access_with_adjusted_size /src/qemu/system/memory.c:573:18
#12 0x5c5e52752dae in memory_region_dispatch_read1 /src/qemu/system/memory.c:0:15
#13 0x5c5e52752dae in memory_region_dispatch_read /src/qemu/system/memory.c:1459:9
#14 0x5c5e52776c5f in flatview_read_continue_step /src/qemu/system/physmem.c:2867:18
#15 0x5c5e52776619 in flatview_read_continue /src/qemu/system/physmem.c:2908:19
#16 0x5c5e5274d527 in address_space_read /src/qemu/include/exec/memory.h:3027:26

This is https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=71649

Thank you