xen: allow disabling PV disks and nics

Introduce two new loader tunnables that can be used to disable PV disks and
PV nics at boot time. They default to 0 and should be set to 1 (or any
number different than 0) in order to disable the PV devices:

hw.xen.disable_pv_disks=1
hw.xen.disable_pv_nics=1

In /boot/loader.conf will disable both PV disks and nics.

Sponsored by: Citrix Systems R&D
Tested by: Karl Pielorz <kpielorz_lst@tdx.co.uk>
MFC after: 1 week