qga/commands-posix: Fix resource leak

It's detected by coverity. Close the dirfd.
Signed-off-by: NShannon Zhao <zhaoshenglong@huawei.com>
Signed-off-by: NShannon Zhao <shannon.zhao@linaro.org>
Reviewed-by: NStefan Weil <sw@weilnetz.de>
Reviewed-by: Nzhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

qga/commands-posix.c

@@ -2283,6 +2283,7 @@ GuestMemoryBlockInfo *qmp_guest_get_memory_block_info(Error **errp)
 
     buf = g_malloc0(20);
     ga_read_sysfs_file(dirfd, "block_size_bytes", buf, 20, &local_err);
+    close(dirfd);
     if (local_err) {
         g_free(buf);
         error_propagate(errp, local_err);