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[email protected]>
Reviewed-by: Dr. David Alan Gilbert <[email protected]>
Reviewed-by: Laurent Vivier <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Thomas Huth <[email protected]>