Mini-OS: get own domid

Mini-OS: get own domid

Get the own domid via creation of a temporary event channel. There is
no "official" way to read the own domid in PV guests, so use the event
channel interface to get it:

- allocate an unbound event channel specifying DOMID_SELF for the
  other end

- read the event channel status which will contain the own domid in
  unbound.dom

- close the event channel

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>