engine: Update host activeNic on GetCapabilities

The patch saves the value of activeNic provided by VDSM property
lastClientIface and returns the vds element which contains it as
a return value of the activate command.

Change-Id: I04e6452078a56e1aba0b57464ffde529285fb34d
Signed-off-by: Moti Asayag <masayag@redhat.com>