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

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

For ARM, the call to arch_domain_create() needs to have completed before
domain_max_vcpus() will return the correct upper bound.

For each arch's dom0's, drop the temporary max_vcpus parameter, and allocation
of dom0->vcpu.

With d->max_vcpus now correctly configured before evtchn_init(), the poll mask
can be constructed suitably for the domain, rather than for the worst-case
setting.

Due to the evtchn_init() fixes, it no longer calls domain_max_vcpus(), and
ARM's two implementations of vgic_max_vcpus() no longer need work around the
out-of-order call.

From this point on, d->max_vcpus and d->vcpus[] are valid for any domain which
can be looked up by domid.

The XEN_DOMCTL_max_vcpus hypercall is modified to reject any call attempt with
max != d->max_vcpus, which does match the older semantics (not that it is
obvious from the code).  The logic to allocate d->vcpu[] is dropped, but at
this point the hypercall still needs making to allocate each vcpu.

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