tests: Fix how qom-test is run

We want to run qom-test for every architecture, without having to
manually add it to every architecture's list of tests.  Commit 3687d53
accomplished this by adding it to every architecture's list
automatically.

However, some architectures inherit their tests from others, like this:

    check-qtest-x86_64-y = $(check-qtest-i386-y)
    check-qtest-microblazeel-y = $(check-qtest-microblaze-y)
    check-qtest-xtensaeb-y = $(check-qtest-xtensa-y)

For such architectures, we ended up running the (slow!) test twice.
Commit 2b8419c attempted to avoid this by adding the test only when
it's not already present.  Works only as long as we consider adding
the test to the architectures on the left hand side *after* the ones
on the right hand side: x86_64 after i386, microblazeel after
microblaze, xtensaeb after xtensa.

Turns out we consider them in $(SYSEMU_TARGET_LIST) order.  Defined as

    SYSEMU_TARGET_LIST := $(subst -softmmu.mak,,$(notdir \
       $(wildcard $(SRC_PATH)/default-configs/*-softmmu.mak)))

On my machine, this results in the oder xtensa, x86_64, microblazeel,
microblaze, i386.  Consequently, qom-test runs twice for microblazeel
and x86_64.

Replace this complex and flawed machinery with a much simpler one: add
generic tests (currently just qom-test) to check-qtest-generic-y
instead of check-qtest-$(target)-y for every target, then run
$(check-qtest-generic-y) for every target.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Message-Id: <1443689999-12182-5-git-send-email-armbru@redhat.com>
