os-posix: change tab to space avoid violating coding style Cc: Eric Blake <eblake@redhat.com> Signed-off-by: Gonglei <arei.gonglei@huawei.com> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>