arm: vgic emulation

arm: vgic emulation

- emulation of the GICD interface for the guest;

- interrupt injection into the guest;

- keep track of inflight irqs using a list, ordered by priority.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Tim Deegan <Tim.Deegan@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>