KVM x86: Extend AMD specific guest behavior to Hygon virtual CPUs

Extend guest_cpuid_is_amd() to cover Hygon virtual CPUs and rename it
accordingly.  Hygon CPUs use an AMD-based core and so have the same
basic behavior as AMD CPUs.

Fixes: b8f4abb6

 ("x86/kvm: Add Hygon Dhyana support to KVM")
Cc: Pu Wen <puwen@hygon.cn>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>