x86/cpuid: Dispatch cpuid_hypervisor_leaves() from guest_cpuid()

x86/cpuid: Dispatch cpuid_hypervisor_leaves() from guest_cpuid()

... rather than from the legacy path.  Update the API to match guest_cpuid(),
and remove its dependence on current.

Make use of guest_cpuid() unconditionally zeroing res to avoid repeated
re-zeroing.  To use a const struct domain, domain_cpuid() needs to be
const-corrected.

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