qemuhotplugtest: Free monitor iff successfully initialized

qemuhotplugtest: Free monitor iff successfully initialized

If initializing test monitor in testQemuHotplugCpuPrepare()
fails, the control jumps to error label where
testQemuHotplugCpuDataFree() is called. But since the data->mon
is NULL due to aforementioned failure,
qemuMonitorTestGetMonitor() dereferences a NULL pointer leading
to a SIGSEGV.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>