xen/arm: vgic-v3: Delay the initialization of the domain information

xen/arm: vgic-v3: Delay the initialization of the domain information

A follow-up patch will require to know the number of vCPUs when
initializating the vGICv3 domain structure. However this information is
not available at domain creation. This is only known once
XEN_DOMCTL_max_vpus is called for that domain.

In order to get the max vCPUs around, delay the domain part of the vGIC
v3 initialization until the first vCPU of the domain is initialized.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Tested-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Acked-but-disliked-by: Stefano Stabellini <sstabellini@kernel.org>
(cherry picked from commit 703d9d5ec13a0f487e7415174ba54e0e3ca158db)