qemu: Check virGetLastError return value for migration finish failure

qemu: Check virGetLastError return value for migration finish failure

Commit id '2e7cea243' added a check for an error from Finish instead
of 'unexpected error'; however, if for some reason there wasn't an
error, then virGetLastError could return NULL resulting in the
NULL pointer deref to err->domain.

(cherry picked from commit ea3c5f25eb94f08d2a54385362c7ad60c6b1854e)