iotests: Add @has_quit to vm.shutdown()

iotests: Add @has_quit to vm.shutdown()

If a test has issued a quit command already (which may be useful to do
explicitly because the test wants to show its effects),
QEMUMachine.shutdown() should not do so again.  Otherwise, the VM may
well return an ECONNRESET which will lead QEMUMachine.shutdown() to
killing it, which then turns into a "qemu received signal 9" line.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>