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

VDI has driver name set to nbd in case of a VM migration failure. 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>