[xen] Set the "feature-rx-notify" flag for netfront devices

[xen] Set the "feature-rx-notify" flag for netfront devices

iPXE already sends RX notifications to the backend when needed, but
does not set the "feature-rx-notify" flag.  As of XenServer 6.5, this
flag is mandatory and omitting it will cause the backend to fail.

Fix by setting the "feature-rx-notify" flag, to inform the backend
that we will send notifications.

Reported-by: Shalom Bhooshi <shalom.bhooshi@citrix.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>