rbd: Use qemu_rbd_connect() in qemu_rbd_do_create()

This is almost exactly the same code. The differences are that
qemu_rbd_connect() supports BlockdevOptionsRbd.server and that the cache
mode is set explicitly.

Supporting 'server' is a welcome new feature for image creation.
Caching is disabled by default, so leave it that way.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>