Added four new hook points for device migrations

Continued effort on device custom properties. Added four new
hook points: before/after_device_migrate_source, and
before/after_device_migrate_destination. The new hooks are called
for every device that has custom properties.

Patch Set 2:
* Added destination hooks, but they aren't called yet
* Modified installation files, man page to reflect new hook points.

Patch Set 3:
* We now call the before/after_device_migrate_destination hooks.
* Since the device._deviceXML cache is not sent when migrating
  a VM, I had to re-populate that cache at the destination host,
  via a search by device alias.

Patch Set 4:
* Fixed wording in man page
* Added more verbose error reporting in _updateDevicesCache
* Updated copyright for libvirtvm.py file

Patch Set 5:
* We now crash horribly if an alias was not found for a device
  when updating devices domxml cache after migrating
* _updateDevicesCache has been changed from O(n^2) to O(n)

Patch Set 6:
* Removed orphaned function
* Semantic changes

Change-Id: I12278b91102f9bc0f031c4605ebf8b43fcb1bd6f
Signed-off-by: Assaf Muller <amuller@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13876
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>