Allow exit code 21 for 'iscsiadm -m session'.

Updates the libvirt volume driver so that it properly handles
exit code 21 in the active session check. This fixes issues
in running the command on Fedora 17.

This behaviour was broken in 933365a (multipath libvirt iscsi).

Fixes LP Bug #1130718.

Change-Id: I80de4c09ba857d550734f9bd0dfc0c40bfd5036a
Merge "Allow exit code 21 for 'iscsiadm -m session'."