CA-127463: After a snapshot failure, VHDs are left in PAUSED state

In the event of a snapshot failure, both tapdisk and VHDs are left in
paused state. During the recovery cycle using journaler mechanism, the
VHDs are brought back to a stable state, but further vdi deactivate will
fail with VHDs in paused state. Currently there is no mechanism to
unpause the VHDs and the tapdisk.

Signed-off-by: Vineeth Thampi Raveendran <vineeth.thampi@citrix.com>
Reviewed-by: Thanos Makatos <thanos.makatos@citrix.com>

GitHub: closes #79 on xapi-project/sm