While debugging a failure of 'virsh qemu-attach', I noticed that
we were leaking the count of active domains on failure.  This
means that a libvirtd session that is supposed to quit after
active domains disappear will hang around forever.

* src/qemu/qemu_process.c (qemuProcessAttach): Undo count of
active domains on failure.
Signed-off-by: NEric Blake <eblake@redhat.com>