block/rbd: increase dynamically the image size

RBD APIs don't allow us to write more than the size set with
rbd_create() or rbd_resize().
In order to support growing images (eg. qcow2), we resize the
image before write operations that exceed the current size.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-id: 20190509145927.293369-1-sgarzare@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>