engine: Raise event log severity to warning

Since IMPORTEXPORT_IMPORT_VM_INVALID_INTERFACES audit log
describes a problem in the system, it should be marked with
a distinguishable severity to reflects its nature better.

Change-Id: I5ec6ab5f48a6ed2e34d976a383685231886a1e93
Bug-Url: http://bugzilla.redhat.com/870121
Signed-off-by: Moti Asayag <masayag@redhat.com>