CA-107356: Fix tapdisk unpause issue in case of VM migration failed.

VDI has driver name set to nbd in case VM migration failed. While
unpause tapdisk, it was always using mirror driver name. In a snapshot
failure scenario, the fix will pass None as driver name to unpause
tapdisk.

Signed-off-by: Sisimon Soman <sisimon.soman@citrix.com>
Reviewed-by: Thanos Makatos <thanos.makatos@citrix.com>

GitHub: closes #27 on xapi-project/sm