qemu.py: improve message on negative exit code

The current message shows 'self._args', which contains only part of the
options used in the Qemu command line.

This patch makes the qemu full args list an instance variable and then
uses it in the negative exit code message.

Message was moved outside the 'if is_running' block to make sure it will
be logged if the VM finishes before the call to shutdown().

Signed-off-by: Amador Pahim <apahim@redhat.com>
Message-Id: <20170901112829.2571-5-apahim@redhat.com>
[ehabkost: removed superfluous parenthesis]
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>