hw/9pfs: avoid buffer overrun

v9fs_add_dir_node and qemu_v9fs_synth_add_file used strncpy
to form node->name, which requires NUL-termination, but
strncpy does not ensure NUL-termination.
Use pstrcpy, which does.

Acked-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
