Disk devices in QEMU have two parts, the guest device and the host
backend driver. Historically these two parts have had the same
"unique" name. With the switch to using -device though, they now
have separate names. Thus when changing CDROM media, for guests
using -device syntax, we need to prepend the QEMU_DRIVE_HOST_PREFIX
constant

* src/qemu/qemu_conf.c, src/qemu/qemu_conf.h: Add helper function
  qemuDeviceDriveHostAlias() for building a host backend alias
* src/qemu/qemu_driver.c: Use qemuDeviceDriveHostAlias() to determine
  the host backend alias for performing eject/change commands in the
  monitor