blockjob: fix memleak that prevented block pivot

blockjob: fix memleak that prevented block pivot

https://bugzilla.redhat.com/show_bug.cgi?id=888426

The code for doing a block-copy was supposed to track the destination
file in drive->mirror, but was set up to do all mallocs prior to
starting the copy so that OOM wouldn't leave things partially started.
However, the wrong variable was being written; later in the code we
silently did 'disk->mirror = mirror' which was still NULL, and thus
leaking memory and leaving libvirt to think that the mirror job was
never started, which prevented a pivot operation after a copy.
Problem introduced in commit 35c7701c6.

* src/qemu/qemu_driver.c (qemuDomainBlockCopy): Initialize correct
variable.