vm_event: Implement ARM SMC events

vm_event: Implement ARM SMC events

The ARM SMC instructions are already configured to trap to Xen by default. In
this patch we allow a user-space process in a privileged domain to receive
notification of when such event happens through the vm_event subsystem by
introducing the PRIVILEGED_CALL type.

The intended use-case for this feature is for a monitor application to be able
insert tap-points into the domU kernel-code. For this task only unconditional
SMC instruction should be used.

Signed-off-by: Tamas K Lengyel <tamas.lengyel@zentific.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>