(2.9) Gracefully handle degraded instances in verification

(2.9) Gracefully handle degraded instances in verification

The current code assumes that every instance either is of type
diskless or has at least one disk. However, with the option to
remove individual disk degraded 0-disk non-diskless instances
can occur. While such instances usually are not useful, Ganeti
still should be operational in their presence.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Hrvoje Ribicic <riba@google.com>
Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>