gnt-* list-fields: Show friendly field type

For people writing query filters (documented in ganeti(7)) knowing a field's
type can be useful.

$ gnt-instance list-fields name be/memory
Name      Type         Title        Description
name      Text         Instance     Instance name
be/memory Storage size ConfigMaxMem The "maxmem" backend parameter

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>