rbd: Do not error out on a single image during pool refresh

rbd: Do not error out on a single image during pool refresh

It could happen that rbd_list() returns X names, but that while
refreshing the pool one of those RBD images is removed from Ceph
through a different route then libvirt.

We do not need to error out in such case, we can simply ignore the
volume and continue.

  error : volStorageBackendRBDRefreshVolInfo:289 :
    failed to open the RBD image 'vol-998': No such file or directory

It could also be that one or more Placement Groups (PGs) inside Ceph
are inactive due to a system failure.

If that happens it could be that some RBD images can not be refreshed
and a timeout will be raised by librados.

  error : volStorageBackendRBDRefreshVolInfo:289 :
    failed to open the RBD image 'vol-893': Connection timed out

Ignore the error and continue to refresh the rest of the pool's
contents.

Signed-off-by: Wido den Hollander <wido@widodh.nl>