xen: add "not_essential" flag to struct xenbus_driver

xen: add "not_essential" flag to struct xenbus_driver

When booting the xenbus driver will wait for PV devices to have
connected to their backends before continuing. The timeout is different
between essential and non-essential devices.

Non-essential devices are identified by their nodenames directly in the
xenbus driver, which requires to update this list in case a new device
type being non-essential is added (this was missed for several types
in the past).

In order to avoid this problem, add a "not_essential" flag to struct
xenbus_driver which can be set to "true" by the respective frontend.

Set this flag for the frontends currently regarded to be not essential
(vkbs and vfb) and use it for testing in the xenbus driver.

Signed-off-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20211022064800.14978-2-jgross@suse.com
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>