xen: Add compatibility aliases for frontend drivers

Before getting merged, xen-blkfront was xenblk and
xen-netfront was xennet.

Temporarily adding compatibility module aliases
eases upgrades from older versions by e.g. allowing
mkinitrd to find the new version of the module.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>