block-vvfat.c: fix warnings with _FORTIFY_SOURCE

block-vvfat.c: fix warnings with _FORTIFY_SOURCE

In function 'snprintf',
    inlined from 'init_directories' at block-vvfat.c:868:10,
    inlined from 'vvfat_open' at block-vvfat.c:1065:24:
/usr/include/bits/stdio2.h:65:3: warning: call to __builtin___snprintf_chk will always overflow destination buffer

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Olaf Hering <olaf@aepfle.de>