rbd: add fallback default poolname 'rbd' to status

rbd: add fallback default poolname 'rbd' to status

the fallback to a default pool name of 'rbd' was introduced in:
1440604a4b072b88cc1e4f8bbae4511b50d1d68e
and worked for the status command, because it used the `rados_cmd`
sub.

This fallback was lost with the changes in:
41aacc6cdeea9b0c8007cbfb280acf827932c3d6

leading to confusing errors:
`Use of uninitialized value in string eq at \
/usr/share/perl5/PVE/Storage/RBDPlugin.pm line 633`
(e.g. in the journal from pvestatd)

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>