qemu.py: don't launch again before shutdown()

If a VM is launched, files are created and a cleanup is required before
a new launch. This cleanup is executed by shutdown(), so shutdown() must
be called even if the VM is manually terminated (i.e. using kill).

This patch creates a control to make sure launch() will not be executed
again if shutdown() is not called after the previous launch().

Signed-off-by: Amador Pahim <apahim@redhat.com>
Message-Id: <20180122205033.24893-7-apahim@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>