KVM: define KVM_CAP_DEVICE_DEASSIGNMENT

define KVM_CAP_DEVICE_DEASSIGNMENT and KVM_DEASSIGN_PCI_DEVICE
for device deassignment.

the ioctl has been already implemented in the
commit: 0a920356

Acked-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Weidong Han <weidong.han@intel.com>
Signed-off-by: Avi Kivity <avi@redhat.com>