Allow snapshots of paused and suspended instances

Fixes bug 1100556

Remove the restriction in the API of snapshotting instances which
are in paused or suspended states and update the libvirt driver
to deal with this accordingly.

Other drivers may need to be updated accordingly.

Change-Id: Iabeb44f843c3c04f767c4103038fcf6c52966ff3
Merge "Allow snapshots of paused and suspended instances"
Merge remote-tracking branch 'citrix/master' into build-temp