gnt-instance info: Rework beparams listings

This patch reworks the beparams listing to the new style. However, due
to this process the naming of the fields with mixed casing changed to
lowercase and is therefore not 100% compatible with the previous versions.

As "memory" is marked deprecated and sort of default we merge that just
into the actual dict, which leads to the following output:

  - maxmem: 128
  - memory: default (128)
  - minmem: 128

Instead of:

  - maxmem: 128
  - minmem: 128
  - memory: 128

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>