tools/libxendevicemodel: add a call to restrict the handle

tools/libxendevicemodel: add a call to restrict the handle

My recent patch [1] to the Linux privcmd module introduced a mechanism
to restrict an open file handle to subsequently only accept operations for
a specified domain.

This patch extends the libxendevicemodel API and make use of the
mechanism in the Linux-specific code to restrict operations on the
interface handle.

[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>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>