ch: Rework virCHProcessConnectMonitor()

Firstly, let's switch from explicit virCHDriverGetConfig() +
virObjectUnref() combo to g_autoptr(virCHDriverConfig). This
leaves us with the @monitor variable which is initialized to NULL
only to be then set to the retval of virCHMonitorNew() and
returned instantly. Well, the variable is now useless and can be
dropped.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>