tests/qtest/migration: Initialize buffer in probe_o_direct_support

Valgrind complains about the probe_o_direct_support() function reading
from an uninitialized buffer. For probing O_DIRECT support we don't
actually need to write to the file, just make sure the pwrite call
doesn't reject the write. Still, write zeroes to the buffer to
suppress the warning.

Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Peter Xu <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Fabiano Rosas <[email protected]>

Author: Fabiano Rosas

tests/qtest/migration/migration-util.c

@@ -284,6 +284,7 @@ bool probe_o_direct_support(const char *tmpfs)
 
     buf = qemu_try_memalign(len, len);
     g_assert(buf);
+    memset(buf, 0, len);
 
     ret = pwrite(fd, buf, len, offset);
     unlink(filename);