xend: allow config file compatibility with new tap syntax

xend: allow config file compatibility with new tap syntax

Recently the format of the tap syntax in the config file changed to
using a 4 part specifier (tap:tapdisk:<format>:<filename>) instead
of the old 3-part one (tap:<qcow>:<filename>).
This breaks compatibility with existing config files: a guest start
will throw a Python exception and will be aborted.
AFAICS currently tap:tapdisk is redundant, so the attached patch
simply catches the above mentioned exception and tries to parse the
old format in this case.

Signed-off-by: Andre Przywara <andre.przywara@amd.com>