engine: MAC Address Format from Guest Agent

This patch changes the MAC addresses that
are reported by the guest agent, replacing the
dashes with colons.

This comes to deal with the guest agent on
Windows, which reports MACs of network interfaces
with dashes.

Change-Id: I76efbbc735957c4ececa82778cfe784b74e3a231
Bug-Url: https://bugzilla.redhat.com/907781
Signed-off-by: Muli Salem <msalem@redhat.com>