Re-factor the hostdev hotplug code so that we can easily add PCI
hostdev hotplug to qemudDomainAttachHostDevice().

* src/qemu_driver.c: rename qemudDomainAttachHostDevice() to
  qemudDomainAttachHostUsbDevice(); make qemudDomainAttachHostDevice()
  handle all hostdev types

* src/libvirt_private.syms: export a couple of hostdev related
  ToString() functions