configs/qemu: enable tmpfs for custom configs

Enable TMPFS support for our custom qemu kernel configs, it's generally
used and can lead to failures/confusion.
Moreover enable ACL & XATTR TMPFS support for SystemD.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>