core: RNG device final touches

This patch deals with 2 small bugs in RNG devices:
1: When creating vm from template, RNG device was created twice (firstly
from AddVmFromTemplateParams and secondly from generic device copying).
Solved by checking presence of RNG device on copying vm devices.

2: When creating vm pool, RNG device couldn't be customized. This was
caused by incorrect RNG propagating in AddVmAndAttachToPoolCommand.

3: Fixed enum parsing in VmRngDevice.

Change-Id: I78fa8f34eeebf02f14130570b581353572b18fad
Bug-Url: https://bugzilla.redhat.com/977079
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>