frontend: Fix issues with VmRngDevice.Source enum

this patch fixes problems with ^ enum parsing in ClusterModel. This enum
is represented as lower-case string in the engine config. When matching
this enum to its string representation, toLowerCase() must be called
firstly.

Change-Id: Ieeb5660fe5da4ea1ba913247150d4c63cecae1bb
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/977079