xen/privcmd: Add IOCTL_PRIVCMD_DM_OP

xen/privcmd: Add IOCTL_PRIVCMD_DM_OP

Recently a new dm_op[1] hypercall was added to Xen to provide a mechanism
for restricting device emulators (such as QEMU) to a limited set of
hypervisor operations, and being able to audit those operations in the
kernel of the domain in which they run.

This patch adds IOCTL_PRIVCMD_DM_OP as gateway for __HYPERVISOR_dm_op.

NOTE: There is no requirement for user-space code to bounce data through
      locked memory buffers (as with IOCTL_PRIVCMD_HYPERCALL) since
      privcmd has enough information to lock the original buffers
      directly.

[1] http://xenbits.xen.org/gitweb/?p=xen.git;a=commit;h=524a98c2

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
OraBug: 26662731

(cherry picked from commit ab520be8cd5d56867fc95cfbc34b90880faf1f9d)
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
Conflicts:
	arch/arm/xen/enlighten.c
	arch/arm/xen/hypercall.S
	arch/arm64/xen/hypercall.S
        (uek doesn't define HYPERVISOR_vm_assist() for ARM)
        arch/arm/include/asm/xen/hypercall.h
        (Upstream defines HYPERVISOR_dm_op() in
         include/xen/arm/hypercall.h, which uek4 does not have)