This patch adds a command-line option (-xen-domid-restrict) which will
use the new libxendevicemodel API to restrict devicemodel [1] operations
to the specified domid. (Such operations are not applicable to the xenpv
machine type).

This patch also adds a tracepoint to allow successful enabling of the
restriction to be monitored.

[1] I.e. operations issued by libxendevicemodel. Operation issued by other
    xen libraries (e.g. libxenforeignmemory) are currently still unrestricted
    but this will be rectified by subsequent patches.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>