sheepdog: avoid a few buffer overruns

* parse_vdiname: Use pstrcpy, not strncpy, when the destination
buffer must be NUL-terminated.
* sd_open: Likewise, avoid buffer overrun.
* do_sd_create: Likewise.  Leave the preceding memset, since
pstrcpy does not NUL-fill, and filename needs that.
* sd_snapshot_create: Add a comment/question.
* find_vdi_name: Remove a useless memset.
* sd_snapshot_goto: Remove a useless memset.
Use pstrcpy to NUL-terminate, because find_vdi_name requires
that its vdi arg (filename parameter) be NUL-terminated.
It seems ok not to NUL-fill the buffer.
Do the same for snapid: remove useless memset-0 (instead,
zero tag[0]).  Use pstrcpy, not strncpy.
* sd_snapshot_list: Use pstrcpy, not strncpy to write
into the ->name member.  Each must be NUL-terminated.

Acked-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
