qapi: Improve include file name reporting in error messages

qapi: Improve include file name reporting in error messages

Error messages print absolute file names of included files even if the
user gave a relative one on the command line:

    $ PYTHONPATH=scripts python -B tests/qapi-schema/test-qapi.py tests/qapi-schema/include-cycle.json
    In file included from tests/qapi-schema/include-cycle.json:1:
    In file included from /work/armbru/qemu/tests/qapi-schema/include-cycle-b.json:1:
    /work/armbru/qemu/tests/qapi-schema/include-cycle-c.json:1: Inclusion loop for include-cycle.json

Improve this to

    In file included from tests/qapi-schema/include-cycle.json:1:
    In file included from tests/qapi-schema/include-cycle-b.json:1:
    tests/qapi-schema/include-cycle-c.json:1: Inclusion loop for include-cycle.json

The error message when an include file can't be opened prints the
include directive's file name, which is relative to the including
file.  Change this to print the file name relative to the working
directory.  Visible in tests/qapi-schema/include-no-file.err.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180211093607.27351-12-armbru@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Eric Blake <eblake@redhat.com>