xen-disk: only advertize feature-persistent if grant copy is not available

If grant copy is available then it will always be used in preference to
persistent maps. In this case feature-persistent should not be advertized
to the frontend, otherwise it may needlessly copy data into persistently
granted buffers.

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