schema: Strengthen schema verification

Add additional checking to process-schema.py to detect the following errors:
 - Symbol comment documents a field which is not defined in the symbol
 - Symbol references an undefined type

Change-Id: Ia750e537fad9ad57d1163906b130033e896c36dd
Signed-off-by: Adam Litke <agl@us.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/11397
Reviewed-by: Saggi Mizrahi <smizrahi@redhat.com>