To enable it to be called from multiple locations, split out
the code for building the -drive arg string. This will be needed
by later patches which do drive hotplug, the conversion to use
-device, and the conversion to controller/bus/unit addressing

* src/qemu/qemu_conf.c, src/qemu/qemu_conf.h: Add qemuBuildDriveStr
  for building -drive arg string