xen/arm: compile and run xenbus

xen/arm: compile and run xenbus

bind_evtchn_to_irqhandler can legitimately return 0 (irq 0): it is not
an error.

If Linux is running as an HVM domain and is running as Dom0, use
xenstored_local_init to initialize the xenstore page and event channel.

Changes in v4:
- do not xs_reset_watches on dom0.

Changes in v2:
- refactor xenbus_init.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
[v5: Fixed case switch indentations]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
(cherry picked from commit ecc635f90adfe1b7cd5fd354f49edfbf24aa4e3e)