xen/domain: Allocate d->vcpu[] earlier during domain_create()

xen/domain: Allocate d->vcpu[] earlier during domain_create()

The ARM code has a chicken-and-egg problem.  One of the vGICv3 emulations
wants to know d->max_vcpus to be able to size itself appropriately, but the
current order of initialisation requires the vGIC to be set up before the
requested number of vcpus can be checked.

Move the range checking of config->max_vcpus into sanitise_domain_config()
path, which allows for the allocation of d->vcpu[] and d->max_vcpus to happen
earlier during create, and in particular, before the call to
arch_domain_create().

The x86 side is fairly easy, and implements the logical equivalent of
domain_max_vcpus() but using XEN_DOMCTL_CDF_hvm_guest rather than
is_hvm_domain().

For the ARM side, re-purpose vgic_max_vcpus() to take a domctl vGIC version,
and return the maximum number of supported vCPUs, reusing 0 for "version not
supported".  To avoid exporting the vgic_ops structures (which are in the
process of being replaced), hard code the upper limits.

This allows for the removal of the domain_max_vcpus() infrastructure, which is
done to prevent it being reused incorrectly in the future.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <julien.grall@arm.com>