arm/xen: add support for vm_assist hypercall

Add support for the Xen HYPERVISOR_vm_assist hypercall.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

arch/arm64/xen/hypercall.S

@@ -82,6 +82,7 @@ HYPERCALL3(vcpu_op);
 HYPERCALL1(tmem_op);
 HYPERCALL1(platform_op_raw);
 HYPERCALL2(multicall);
+HYPERCALL2(vm_assist);
 
 ENTRY(privcmd_call)
 	mov x16, x0