xen/dom0: Arrange for dom0_cfg to contain the real max_vcpus value

xen/dom0: Arrange for dom0_cfg to contain the real max_vcpus value

Make dom0_max_vcpus() a common interface, and implement it on ARM by splitting
the existing alloc_dom0_vcpu0() function in half.

As domain_create() doesn't yet set up the vcpu array, the max value is also
passed into alloc_dom0_vcpu0().  This is temporary for bisectibility and
removed in the following patch.

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