x86: modify setup_dom0_vcpu to use dom0_cpus internally

x86: modify setup_dom0_vcpu to use dom0_cpus internally

We will later move dom0 builders to different directories. To avoid the
need of making dom0_cpus visible outside dom0_builder.c, modify
setup_dom0_vcpus to cycle through dom0_cpus internally instead of
relying on the callers to do that.

No functional change.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>