Make sure domain exists before referencing it

Before calling domain.destroy() in the exception handler for a failed
create attempt, make sure domain was created prior to when the
exception was thrown.

Conflicts:
        nova/virt/libvirt/driver.py

Change-Id: I3b7c5afce7f3bacce1b444c245b035334962ff7c
Closes-Bug: #1326998
(cherry picked from commit 5a98983e87e5020626db2e319f7b00ae547ae8e1)
Merge "Make sure domain exists before referencing it" into stable/icehouse