If readdir_r fails, error_setg_errno will reference the freed
pointer *dirpath*.

Moreover, readdir_r may cause a buffer overflow, using readdir instead.
Signed-off-by: Nzhanghailiang <zhang.zhanghailiang@huawei.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>