api: Organize schema symbols by type

Currently the vdsmapi module returns the parsed schema as a large list of
symbols.  Consumers of this list have all resorted to reorganizing the symbols
by type (commands, types, enums, maps, etc).  Rather than forcing each user to
rewrite this code, just do it correctly at the source.  This reduces code
duplication and makes at easier to add new symbol types (such as events and
errors).

Signed-off-by: Adam Litke <agl@us.ibm.com>
Change-Id: Ie20291689c402e3f49267181eeb96f2cedbe9690
Reviewed-on: http://gerrit.ovirt.org/12106
Reviewed-by: Saggi Mizrahi <smizrahi@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>