tests/qtest/migration: Fix cpr-tests in case the machine is not available

tests/qtest/migration: Fix cpr-tests in case the machine is not available

When QEMU has been compiled with "--without-default-devices", the
migration cpr-tests are currently failing since the first test leaves
a socket file behind that avoids that the second test can be initialized
correctly. Make sure that we delete the socket file in case that the
migrate_start() failed due to the missing machine.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250930090932.235151-1-thuth@redhat.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>