Now that 4.2 and earlier are no longer supported "xc_interface *" is
always the right type for the xc interface handle.

With this we can also simplify the handling of the xenforeignmemory
compatibility wrapper by making xenforeignmemory_handle ==
xc_interface, instead of an xc_interface* and remove various uses of &
and *h.
Signed-off-by: NIan Campbell <ian.campbell@citrix.com>
Reviewed-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>