Refinements to the --libxo support for geom status and list sub commands.

Changes based on comments in D53110: tags should be lowercase; rename a
few containers so that the JSON/XML output says "DISK" or "MULTIPATH"
(depending on class) instead of a generic "Geom"; adds {t:} to trim
extra whitespaces that sometimes appeared in the value fields of
JSON/XML output.

Submitted-by: Johan Söllvander
MFC-after: 1 week
Differential Revision: https://reviews.freebsd.org/D53313