gnttab_request_version() always sets the gnttab_interface variable
and the assertions to check for empty gnttab_interface is unnecessary.
The patch eliminates multiple such assertions.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>