libxl/xl: drop support for netchannel2

libxl/xl: drop support for netchannel2

netchannel2 was never widely deployed and no supported kernel includes
either the front- or back-ends. The last known kernel with this
support was the xen.git 2.6.31 branch which has been unsupported for
ages.

xl will warn the user if it spots a "vif2" configration item but
otherwise support is completely removed.

Work is ongoing to add the interesting features of netchannel2 as
protocol extensions to netchannel1.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>