engine: check if vds exists with same name

Adds some checking to AddVdsCommand an updates UpdateVdsCommand to check
if there is such a VDS in the database already.

Change-Id: I59bea31c78da3f718497bd1b58bada21c744fdc7
Bug-Url: https://bugzilla.redhat.com/893089
Signed-off-by: Laszlo Hornyak <lhornyak@redhat.com>