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>