qemu_driver: validate mem->model on MEMORY_DEVICE_SIZE_CHANGE event

qemu_driver: validate mem->model on MEMORY_DEVICE_SIZE_CHANGE event

When guest acknowledges change in size of virtio-mem (portion
that's exposed to the guest), QEMU emits
MEMORY_DEVICE_SIZE_CHANGE event. We process it in
processMemoryDeviceSizeChange(). So far, QEMU emits the even only
for virtio-mem (as that's the only memory device model that
allows live changes to its size). Nevertheless, if this ever
changes, validate the memory model upon processing the event as
the rest of the code blindly expects virtio-mem model.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>