[libvirt] [RFC 1/5]: Return exitstatus from virStorageBackendRunProgRegex

This patch changes things around so that virStorageBackendRunProgRegex() does *not* virStorageReportError() if the fork()/exec() process it spawned returned a != 0 exit code. Rather, it returns the exitcode in this case, and it is up to the higher level to determine whether this is a fatal error or not. The use case for this change is in the iSCSI stuff; older versions of iscsiadm tools would return a failure when getting the session number, despite the command succeeding. Signed-off-by: Chris Lalancette <clalance(a)redhat.com&gt;