introduce domain_max_vcpus() helper and implement per arch

introduce domain_max_vcpus() helper and implement per arch

This allows the common XEN_DOMCTL_max_vcpus handler to lose some x86-specific
architecture knowledge.

It turns out that Xen had the same magic number twice in-tree with different
names (HVM_MAX_VCPUS and MAX_HVM_VCPUS).  This removes all use of
MAX_HVM_VCPUS, and x86 uses HVM_MAX_VCPUS from the public headers.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>