interface: remove global manifest_descs list

The list was global and had no locking.  It's not like we were ever
parsing more than one manifest at the same time right now, but we might
in the future.  And we really want this to be local to the interface
itself, for future work redoing how to bind protocols to bundles, so
move the list to the interface structure.

Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>