engine: Frontend support for setting per-VM VncKeyboardLayout

Added combobox to EditVM dialog for setting the VncKeyboardLayout per
individual VM.

Change-Id: I8f143e5b8425805ea87651ec382f0bc95f584591
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=916866
Signed-off-by: Martin Betak <mbetak@redhat.com>