selftests: kvm: introduce cpu_has_svm() check

Many tests will want to check if the CPU is Intel or AMD in
guest code, add cpu_has_svm() and put it as static
inline to svm_util.h.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20200529130407.57176-1-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>