qapi script: remember line number in schema parsing

qapi script: remember line number in schema parsing

Before this patch, 'QAPISchemaError' scans whole input until 'pos'
to get error line number. After this patch, the scan is avoided since
line number is remembered in schema parsing. This patch also benefits
other error report functions, which would be introduced later.

Signed-off-by: Wenchao Xia <wenchaoqemu@gmail.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>