libxl: support getting and setting parameters for the Credit2

libxl: support getting and setting parameters for the Credit2

With Credit2 being Xen default scheduler, it's definitely the case to
allow Credit2's scheduling parameters to be get and set via libvirt.

This is easy, as Credit and Credit2 have (at least as of now) the very
same parameters ('weight' and 'cap'). So we can just let credit2 pass
the scheduler-type check and the same code will work for both.

Signed-off-by: Dario Faggioli <dfaggioli@suse.com>
Reviewed-by: Jim Fehlig <jfehlig@suse.com>