Fix QEMU hotplug device alias assignment

Fix QEMU hotplug device alias assignment

To allow devices to be hot(un-)plugged it is neccessary to ensure
they all have a unique device aliases. This fixes the hotplug
methods to assign device aliases before invoking the monitor
commands which need them

* src/qemu/qemu_conf.c, src/qemu/qemu_conf.h: Expose methods
  for assigning device aliases for disks, host devices and
  controllers
* src/qemu/qemu_driver.c: Assign device aliases when hotplugging
  all types of device
* tests/qemuxml2argvdata/qemuxml2argv-hostdev-pci-address-device.args,
  tests/qemuxml2argvdata/qemuxml2argv-hostdev-usb-address-device.args:
  Update for changed hostdev naming scheme