util: switch to use the GLib event loop impl

util: switch to use the GLib event loop impl

This sets the GLib event loop as the impl when calling
virEventRegisterDefaultImpl(). This remains a private
impl detail of libvirt, so applications must *NOT*
assume that a call to virEventRegisterDefaultImpl()
results in a GLib based event loop.

They should continue to use the libvirt-glib API
gvir_event_register() if they explicitly want to guarantee
a GLib event loop.

This follows the general principal that the libvirt public
API should not expose the fact that GLib is being used
internally.

Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>