x86/hvm: Use guest_cpuid() rather than hvm_cpuid()

x86/hvm: Use guest_cpuid() rather than hvm_cpuid()

More work is required before maxphysaddr can be read straight out of the
cpuid_policy block, but in the meantime hvm_cpuid() wants to disappear so
update the code to use the newer interface.

Use the behaviour of max_leaf handling (returning all zeros) to avoid a double
call into guest_cpuid().

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