block/qcow.c: fix warnings with _FORTIFY_SOURCE

block/qcow.c: fix warnings with _FORTIFY_SOURCE

CC    block/qcow.o
cc1: warnings being treated as errors
block/qcow.c: In function 'qcow_create':
block/qcow.c:804: error: ignoring return value of 'write', declared with attribute warn_unused_result
block/qcow.c:806: error: ignoring return value of 'write', declared with attribute warn_unused_result
block/qcow.c:811: error: ignoring return value of 'write', declared with attribute warn_unused_result
make: *** [block/qcow.o] Error 1

Signed-off-by: Kirill A. Shutemov <[email protected]>
Signed-off-by: Juan Quintela <[email protected]>
Signed-off-by: Anthony Liguori <[email protected]>