virsh: fix memory leak in cmdVolPath code

virsh: fix memory leak in cmdVolPath code

* tools/virsh.c: avoid memory leak in cmdVolPath.
* src/libvirt.c: Add doc for virStorageVolGetPath to tell one
  must free() the returned path after use.

* how to reproduce?

% dd if=/dev/zero of=/var/lib/libvirt/images/foo.img count=1 bs=10M
% virsh pool-refresh default
% valgrind -v --leak-check=full virsh vol-path --vol \
/var/lib/libvirt/images/foo.img

* actual results:

Detected in valgrind run:

==16436== 32 bytes in 1 blocks are definitely lost in loss record 7 of 22
==16436==    at 0x4A05FDE: malloc (vg_replace_malloc.c:236)
==16436==    by 0x386A314B3D: xdr_string (in /lib64/libc-2.12.so)
==16436==    by 0x3DF8CD770D: xdr_remote_nonnull_string (remote_protocol.c:3
==16436==    by 0x3DF8CD7EC8: xdr_remote_storage_vol_get_path_ret
% virsh pool-refresh default
% valgrind -v --leak-check=full virsh vol-path --vol \
/var/lib/libvirt/images/foo.img

Signed-off-by: Alex Jia <ajia@redhat.com>