qemu: attach: Close monitor socket on connection failure

qemu: attach: Close monitor socket on connection failure

If attaching to a qemu process fails after opening the monitor socket
libvirt does not clean up the monitor. As the monitor also holds a
reference to the domain object the qemu attach API basically leaks it.

QEMU also does not interact on a second monitor connection and thus a
further attempt to attach to it would lock up.

Prevent libvirt from leaking the monitor by explicitly closing it.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1378401