tools/libxenforeignmemory: add a call to restrict the handle

tools/libxenforeignmemory: add a call to restrict the handle

Commit 8ef5f344d061 "tools/libxendevicemodel: add a call to restrict the
handle" added a function to the devicemodel interface to restrict
operations through the API to a specific domain, where a capable under-
lying privcmd driver exists.

This patch adds similar functionality to the xenforeignmemory API. This
will be necessary (as much as xendevicemodel restriction) for limiting
the scope of device models to specific domains.

NOTE: My patch to the linux kernel [1] added the appropriate checks to
      the foreign memory ioctls.

[1] https://git.kernel.org/cgit/linux/kernel/git/ostr/linux.git/commit/?id=4610d240

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>