tests/migration: Disambiguate guestperf vs. a-b

tests/migration: Disambiguate guestperf vs. a-b

The current build structure for migration tests is confusing. There is
the tests/migration directory, which contains two different guest code
implementations, one for the qtests (a-b-{bootblock|kernel}.S) and
another for the guestperf script (stress.c). One uses a Makefile,
while the other uses meson.

The next patches will add a new qtests/migration/ directory to hold
qtest code which will make the situation even more confusing.

Move the guest code used by qtests into a new qtests/migration/
directory and rename the old one to tests/migration-stress.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>