KVM: x86: Add callback to let modules decide over some supported cpuid bits

This patch adds the get_supported_cpuid callback to
kvm_x86_ops. It will be used in do_cpuid_ent to delegate the
decission about some supported cpuid bits to the
architecture modules.

Cc: stable@kernel.org
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Avi Kivity <avi@redhat.com>