xl: Add defaultbridge config option for xl.conf

xl: Add defaultbridge config option for xl.conf

Currently guests created with the xl stack will have "xenbr0"
written as their default into xenstore. It can be changed in
the individual guest config files, but there is no way to
have that default globally changed.

Add a config option to xl.conf that allows to have a different
default bridge name.

Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>