xen/arm: vgic: Keep track of vIRQ used by a domain

xen/arm: vgic: Keep track of vIRQ used by a domain

While it's easy to know which hardware IRQ is assigned to a domain, there
is no way to know which vIRQ is allocated by Xen for a specific domain.

Introduce a bitmap to keep track of every vIRQ used by a domain. This
will be used later to find free vIRQ for interrupt device assignment and
emulated interrupt.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>