virtio-gpu: block migration of VMs with blob=true

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>
(cherry picked from commit 9c549ab6895a43ad0cb33e684e11cdb0b5400897)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>