If the q35 specific disable s3/s4 setting isn't supported, fallback to
specifying the PIIX setting, which is the previous behavior. It doesn't
have any effect, but qemu will just warn about it rather than error:

  qemu-system-x86_64: Warning: global PIIX4_PM.disable_s3=1 not used
  qemu-system-x86_64: Warning: global PIIX4_PM.disable_s4=1 not used

Since it doesn't error, I don't think we should either, since there
may be configs in the wild that already have q35 + disable_s3/4 (via
virt-manager)