qtest: Prepare QOM machine tests

Instantiate all [*] machines per target, so that they get a bit of test
coverage at all. This has proven helpful during QOM refactorings.

[*] ppcemb target contains some non-working non-embedded machines, and
ppc405 CPUs are not available there either.
i386 and x86_64 do not cover pc*-x.y or xenfv.

Signed-off-by: Andreas Färber <afaerber@suse.de>