qemurunner.py: fix error message about qmp

The error message is a little misleading as the qmp module is a
directory with __init__.py file, not qmp.py file. Also, put the
path where we try to import it from in the error message to make
the message more indicative.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>