virtio pmem: remove memdev null check

Coverity reports that when we're assigning vi->size we handle the
"pmem->memdev is NULL" case; but we then pass it into
object_get_canonical_path(), which unconditionally dereferences it
and will crash if it is NULL. If this pointer can be NULL then we
need to do something else here.

We are removing 'pmem->memdev' null check here as memdev will never
be null in this function.

Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
Message-Id: <20190712073554.21918-3-pagupta@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>