hw/9pfs: Fix memory leak in error path

Fix few more memory leaks in virtio-9p-device.c detected using valgrind.

Signed-off-by: M. Mohan Kumar <mohan@in.ibm.com>
Message-id: 1372929678-14341-1-git-send-email-mohan@in.ibm.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>