tests/qtest/migration: Move qmp helpers to a separate file

tests/qtest/migration: Move qmp helpers to a separate file

We current have a bunch of non-test functions in migration-test.c and
some others in migration-helpers.c. In order to split migration-test.c
into separate test binaries, these helpers need to go somewhere
else.

To avoid making migration-helpers even larger, move all QMP-related
functions into a new migration-qmp.c file and put it under the
qtest/migration/ directory.

The new file holds everything that has as its main responsibility to
call into QMP.

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