kvm tools, 9p: Fix memory leak in virtio_p9_walk()

kvm tools, 9p: Fix memory leak in virtio_p9_walk()

This patch fixes the following valgrind reported memory leak in
virtio_9p_walk():

  ==6689== 1,058 bytes in 134 blocks are definitely lost in loss record 39 of 47
  ==6689==    at 0x4A0649D: malloc (vg_replace_malloc.c:236)
  ==6689==    by 0x4116A9: virtio_p9_pdu_readf (9p-pdu.c:110)
  ==6689==    by 0x40FB9F: virtio_p9_walk (9p.c:305)
  ==6689==    by 0x410E73: virtio_p9_do_io (9p.c:1151)
  ==6689==    by 0x40DB45: thread_pool__threadfunc (threadpool.c:56)
  ==6689==    by 0x3DE7007B30: start_thread (in /lib64/libpthread-2.14.so)
  ==6689==    by 0x3DE68DFD2C: clone (in /lib64/libc-2.14.so)

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