kvm tools, 9p: Fix memory leak in virtio_9p_attach()

kvm tools, 9p: Fix memory leak in virtio_9p_attach()

This patch fixes the following valgrind reported memory leak:

  ==7684== 20 bytes in 8 blocks are definitely lost in loss record 3 of 46
  ==7684==    at 0x4A0649D: malloc (vg_replace_malloc.c:236)
  ==7684==    by 0x4116B5: virtio_p9_pdu_readf (9p-pdu.c:110)
  ==7684==    by 0x40FD8C: virtio_p9_attach (9p.c:354)
  ==7684==    by 0x410E80: virtio_p9_do_io (9p.c:1154)
  ==7684==    by 0x40DB45: thread_pool__threadfunc (threadpool.c:56)
  ==7684==    by 0x3DE7007B30: start_thread (in /lib64/libpthread-2.14.so)
  ==7684==    by 0x3DE68DFD2C: clone (in /lib64/libc-2.14.so)

Signed-off-by: Pekka Enberg <penberg@kernel.org>