VM element: Enable serial console on Debian
VM element: Enable serial console on Debian

Debian based distros do not enable the serial console by default
in grub.cfg, instead sending all boot messages to the display

Change-Id: I0e80859198e77862ab0301ab27ebe11702fba09d

switch-ch/diskimage-builder,takahashinobuyuki/diskimage-builder,switch-ch/diskimage-builder,varunarya10/diskimage-builder,eantyshev/diskimage-builder,varunarya10/diskimage-builder,rdo-management/diskimage-builder,takahashinobuyuki/diskimage-builder,zioproto/diskimage-builder,NaohiroTamura/diskimage-builder,zioproto/diskimage-builder,rdo-management/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,openstack/diskimage-builder,JioCloud/diskimage-builder,JioCloud/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,openstack/diskimage-builder,eantyshev/diskimage-builder,NaohiroTamura/diskimage-builder