qemu: Fix auto-shutdown of qemu VMs by the qemu driver

qemu: Fix auto-shutdown of qemu VMs by the qemu driver

When auto-shutdown via the qemu driver is requested (rather than via
libvirt guests) we need to start the VMs in a way that they will be kept
around for libvirt to terminate them. This involves inverting the
dependancy relationship for the machined unit file.

Since the setup is done at startup of the VM, add a disclaimer to
qemu.conf that switching between the two modes with VMs running will not
work properly.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>