xen/privcmd: add IOCTL_PRIVCMD_RESTRICT

The purpose if this ioctl is to allow a user of privcmd to restrict its
operation such that it will no longer service arbitrary hypercalls via
IOCTL_PRIVCMD_HYPERCALL, and will check for a matching domid when
servicing IOCTL_PRIVCMD_DM_OP or IOCTL_PRIVCMD_MMAP*. The aim of this
is to limit the attack surface for a compromised device model.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>