virtio-mmio: fix NULL pointer dereference

The callbacks array is not used, so the virtio-serial driver passes
a NULL pointer.  But vm_find_vqs dereferences it (and names too)
unconditionally.  Wrap the accesses with a conditional.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>