tests/qtest/migration-test: Use g_autofree to avoid leaks on error paths

tests/qtest/migration-test: Use g_autofree to avoid leaks on error paths

Coverity notices that several places in the migration-test code fail
to free memory in error-exit paths.  This is pretty unimportant in
test case code, but we can avoid having to manually free the memory
entirely by using g_autofree.

The places where Coverity spotted a leak were relating to early exits
not freeing 'uri' in test_precopy_unix(), do_test_validate_uuid(),
migrate_postcopy_prepare() and test_migrate_auto_converge().  This
patch converts all the string-allocation in the test code to
g_autofree for consistency.

Fixes: Coverity CID 1432313, 1432315, 1432352, 1432364
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20210506185819.9010-1-peter.maydell@linaro.org>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>