xen: allow global VIRQ handlers to be delegated to other domains

xen: allow global VIRQ handlers to be delegated to other domains

This patch sends global VIRQs to a domain designated as the VIRQ
handler
instead of sending all global VIRQ events to dom0. This is required in
order to run xenstored in a stubdom, because VIRQ_DOM_EXC must be sent
to xenstored for domain destruction to work properly.

This patch was inspired by the xenstored stubdomain patch series sent
to xen-devel by Alex Zeffertt in 2009.

Signed-off-by: Diego Ongaro <diego.ongaro@citrix.com>
Signed-off-by: Alex Zeffertt <alex.zeffertt@eu.citrix.com>
Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Keir Fraser <keir@xen.org>