tests/qtest/migration: Move bootfile code to its own file

tests/qtest/migration: Move bootfile code to its own file

Move the code that creates the guest binary out of migration-test and
into the qtest/migration/ directory, along with the rest of the
a-b-kernel code.

That code is part of the basic infrastructure of migration tests, it
shouldn't be among the tests themselves.

Also take the chance and rename migration-test.h, which is too generic
a name for this header which contains only values related to guest
memory offsets.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>