virtio-gpu: block migration of VMs with blob=true
"blob" resources don't have an associated pixman image:
#0 pixman_image_get_stride (image=0x0) at ../pixman/pixman-image.c:921
#1 0x0000562327c25236 in virtio_gpu_save (f=0x56232bb13b00, opaque=0x56232b555a60, size=0, field=0x5623289ab6c8 <__compound_literal.3+104>, vmdesc=0x56232ab59fe0) at ../hw/display/virtio-gpu.c:1225
Related to:
https://bugzilla.redhat.com/show_bug.cgi?id=
2236353
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>