To allow for better code reuse from hotplug methods, the code for
generating PCI/USB hostdev arg values is split out into separate
methods

* qemu/qemu_conf.h, qemu/qemu_conf.c: Introduce new APis for
  qemuBuildPCIHostdevPCIDevStr, qemuBuildUSBHostdevUsbDevStr
  and qemuBuildUSBHostdevDevStr