block/vvfat.c: fix warnings with _FORTIFY_SOURCE

CC    block/vvfat.o
cc1: warnings being treated as errors
block/vvfat.c: In function 'commit_one_file':
block/vvfat.c:2259: error: ignoring return value of 'ftruncate', declared with attribute warn_unused_result
make: *** [block/vvfat.o] Error 1
  CC    block/vvfat.o
In file included from /usr/include/stdio.h:912,
                 from ./qemu-common.h:19,
                 from block/vvfat.c:27:
In function 'snprintf',
    inlined from 'init_directories' at block/vvfat.c:871,
    inlined from 'vvfat_open' at block/vvfat.c:1068:
/usr/include/bits/stdio2.h:65: error: call to __builtin___snprintf_chk will always overflow destination buffer
make: *** [block/vvfat.o] Error 1

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>
