qemu: Add missing lock in qemuProcessHandleMonitorEOF

qemuMonitorUnregister will be called in multiple threads (e.g. threads
in rpc worker pool and the vm event thread).  In some cases, it isn't
protected by the monitor lock, which may lead to call g_source_unref
more than one time and a use-after-free problem eventually.

Add the missing lock in qemuProcessHandleMonitorEOF (which is the only
position missing lock of monitor I found).

Suggested-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Peng Liang <liangpeng10@huawei.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>