engine: add logging for VM NIC link up/down

When link is switched UP/DOWN on VM NIC it should be logged on AuditLog.
This patch expands the NETWORK_UPDATE_VM_INTERFACE message to include
information about changes in the link state.

Change-Id: I3e7d34d8e7c213e4300f9405e90c4fba298f665d
Bug-Url: https://bugzilla.redhat.com/902692
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>