Create GuestAgent instance in __init__ and connect later

This is not only a cosmetic improvement. There are cases where we
are trying to call methods of the GuestAgent before the instance was
created. To avoid these race conditions we're creating the instance of
the guest agent already in the __init__ phase.

Change-Id: I82f7397b01bff48a3c635eee9912cc67cf722b13
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26142
Reviewed-by: Dan Kenigsberg <danken@redhat.com>