tools/blktap2: fix possible '\0' truncation

tools/blktap2: fix possible '\0' truncation

gcc-8 complains:

    tapdisk-vbd.c: In function 'tapdisk_vbd_resume_ring':
    tapdisk-vbd.c:1671:53: error: 'snprintf' output may be truncated before the last format character [-Werror=format-truncation=]
       snprintf(params.name, sizeof(params.name) - 1, "%s", message);
                                                         ^
    tapdisk-vbd.c:1671:3: note: 'snprintf' output between 1 and 256 bytes into a destination of size 255
       snprintf(params.name, sizeof(params.name) - 1, "%s", message);
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The "- 1" in buffer size should be actually applied to message, to leave
place for terminating '\0', not the other way around (truncate '\0' even
if it would fit).

    In function 'tapdisk_control_open_image',
        inlined from 'tapdisk_control_handle_request' at tapdisk-control.c:660:10:
    tapdisk-control.c:465:2: error: 'strncpy' specified bound 256 equals destination size [-Werror=stringop-truncation]
      strncpy(params.name, vbd->name, BLKTAP2_MAX_MESSAGE_LEN);
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    In function 'tapdisk_control_create_socket',
        inlined from 'tapdisk_control_open' at tapdisk-control.c:836:9:
    tapdisk-control.c:793:2: error: 'strncpy' specified bound 108 equals destination size [-Werror=stringop-truncation]
      strncpy(saddr.sun_path, td_control.path, sizeof(saddr.sun_path));
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    block-qcow.c: In function 'qcow_create':
    block-qcow.c:1216:5: error: 'strncpy' specified bound 4096 equals destination size [-Werror=stringop-truncation]
         strncpy(backing_filename, backing_file,
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          sizeof(backing_filename));
          ~~~~~~~~~~~~~~~~~~~~~~~~~

I those cases, reduce size of copied string and make sure final '\0' is
added.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-Acked-by: Juergen Gross <jgross@suse.com>