KVM: selftests: Move VMX_EPT_VPID_CAP_AD_BITS to vmx.h

This is a VMX-related macro so move it to vmx.h. While here, open code
the mask like the rest of the VMX bitmask macros.

No functional change intended.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20220520233249.3776001-6-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>