core: Added validation for bond name format

Added enforcement for bond names of the form "^bond\d*$", the format
assumed and used by VDSM. This is to enable adding new bonds from the
engine.

Change-Id: Id4ea20997c03c35c1f47d9ab12a49a39dd9c778a
Signed-off-by: Lior Vernia <lvernia@redhat.com>