qemuMonitorIO: Don't use @mon after it's unrefed

qemuMonitorIO: Don't use @mon after it's unrefed

https://bugzilla.redhat.com/show_bug.cgi?id=1018267

The aim of virObject refing and urefing is to tell where the object is
to be used and when is no longer needed. Hence any object shouldn't be
used after it has been unrefed, as we might be the last to hold the
reference. The better way is to call virObjectUnref() *after* the last
object usage. In this specific case, the monitor EOF handler was called
after the qemuMonitorIO called virObjectUnref. Not only that @mon was
disposed (which is not used in the handler anyway) but the @mon->vm
which is causing a SIGSEGV:

2013-11-15 10:17:54.425+0000: 20110: error : qemuMonitorIO:688 : internal error: early end of file from monitor: possible problem:
qemu-kvm: -incoming tcp:01.01.01.0:49152: Failed to bind socket: Cannot assign requested address

Program received signal SIGSEGV, Segmentation fault.
qemuProcessHandleMonitorEOF (mon=<optimized out>, vm=0x7fb728004170) at qemu/qemu_process.c:299
299         if (priv->beingDestroyed) {
(gdb) p *priv
Cannot access memory at address 0x0
(gdb) p vm
$1 = (virDomainObj *) 0x7fb728004170
(gdb) p *vm
$2 = {parent = {parent = {magic = 3735928559, refs = 0, klass = 0xdeadbeef}, lock = {lock = {__data = {__lock = 2, __count = 0, __owner = 20110, __nusers = 1, __kind = 0, __spins = 0, __list = {__prev = 0x0,
            __next = 0x0}}, __size = "\002\000\000\000\000\000\000\000\216N\000\000\001", '\000' <repeats 26 times>, __align = 2}}}, pid = 0, state = {state = 0, reason = 0}, autostart = 0, persistent = 0,
  updated = 0, def = 0x0, newDef = 0x0, snapshots = 0x0, current_snapshot = 0x0, hasManagedSave = false, privateData = 0x0, privateDataFreeFunc = 0x0, taint = 304}

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>