Clarify controllers -device string in QEMU driver

Clarify controllers -device string in QEMU driver

The QEMU driver contained code to generate a -device string for piix4-ide, but
wasn't using it. This change removes this string generation. It also adds a
comment explaining why IDE and FDC controllers don't generate -device strings.

The change also generates an error if a sata controller is specified for a QEMU
domain, as this isn't supported.

* src/qemu/qemu_conf.c: Remove VIR_DOMAIN_CONTROLLER_TYPE_IDE handler in
  qemuBuildControllerDevStr(). Ignore IDE and FDC controllers. Error if
  SATA controller is discovered. Add comments.