tests/migration: Do not use functions anymore that rely on global_qtest

tests/migration: Do not use functions anymore that rely on global_qtest

The migration tests deal with multiple test states, so we really should
not use functions here that rely on the single global_qtest variable.
Switch from qtest_start() to qtest_init() to make sure that global_qtest
is not set anymore. This also revealed a regression in the migrate()
function: It has once been converted to use the qtest_qmp() function,
but commit b5bbd3f315d686bd511 ("Clean up string interpolation into QMP,
part 2") accidentally reverted it back to qmp().

Message-Id: <20190904130047.25808-2-thuth@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>