RHEVm - Provide for user selectable first boot device.

The default first boot device has been changed in the rbovirt gem
to hard disk, followed by network. This push allows the user to
explicitly specify the first boot device as either netowrk or
hard disk with the other becoming the second boot device.
TrackedAt: http://tracker.deltacloud.org/patch/76cf9d5d5f4b72e42f125af39327ea7d9cae4734
Bump rbovirt version to 0.0.19 in gemspec

TrackedAt: http://tracker.deltacloud.org/patch/378f9923509139e975a9775243dfdf6d5eefd1f4