Add Xen function to get and set PV_ARGS
Add Xen function to get and set PV_ARGS

PV_ARGS are used to pass additional commandline arguments to the
kernel when running XenPV domains.

The new actions can be run as follows:

salt-cloud -a get_pv_args xenvm01
salt-cloud -a set_pv_args xenvm01 pv_args="utf-8 graphical"

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt