xen: flag pvcalls-front to be not essential for system boot

The Xen pvcalls device is not essential for booting. Set the respective
flag.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20211022064800.14978-5-jgross@suse.com


Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>