xend: allow pv_ops kernel driver pci-stub to hide devices for assignment

xend: allow pv_ops kernel driver pci-stub to hide devices for assignment

pciback is used by VT-d to hide device for assigment. But in pv-ops
dom0, pciback is not supported yet. Fortunately, pci-stub module is
used to hide device in Linux for KVM VT-d device assignment and it's
included in pv-ops dom0. So can use pci-stub to hide devices for
assignment.

Device must be hidden before assignment. Control panel has checks if
devices can be assigned or not, and can list assignable devices via
reading devices owned by pciback. This patch changes the checks, and
also list assignable devices which are owned by pci-stub. Use pci-stub
to hide devices, and use this patch to pass checkes in control panel,
device assignemnt with VT-d works on Xen with pv-ops dom0.

Signed-off-by: Weidong Han <weidong.han@intel.com>