acpi-test: basic acpi unit-test

We run bios, and boot a minimal boot sector that immediately halts.
Then poke at memory to find ACPI tables.

This only checks that RSDP is there.
More will be added later.

Cc: Andreas Färber <afaerber@suse.de>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
