If there is IPv6 on both interface, Linux will put a default route on
ens3 preventing IPv6 to work correctly on ens4.
Even disabling totally IPv6 from inside the host on ens3 may not work.
It is safer to disable it from qemu process directly. Also, it will ease
the configuration of the host.

When the image is a block device, the guest must also run Linux >= 5.0.

otherwise it will stall VM execution.

This is observed inside qemu VM started by 

https://lab.nexedi.com/nexedi/slapos/blob/master/software/erp5testnode/testsuite/deploy-test/software.cfg

In qemu log one can see:
===
QEMU waiting for connection on: disconnected:unix:/srv/slapgrid/slappart6/var/qmp_socket,server

===

Affected version:

root@z600:/srv/slapgrid/slappart6# /opt/slapgrid/7903668a197f3cf8f94375d0739a21e8/parts/kvm/bin/qemu-system-x86_64 --version
QEMU emulator version 2.8.0
Copyright (c) 2003-2016 Fabrice Bellard and the QEMU Project developer

/reviewed-on nexedi/slapos!585

Close the NBD socket when testing it so it won't mess up qemu.

/reviewed-on nexedi/slapos!516

Set cache.direct=on when using native aio (needed by qemu)

/reviewed-on nexedi/slapos!399

/reviewed-on nexedi/slapos!380

  it  clean up....

  Up to some extent we would like to set diskcache=none and diskaio=native

Allow to Add CPU or RAM in hotplug mode, without need of restart qemu process

If restrict mode is set to true, nat interface (eth0) will be isolated, no network access, only host and guest forward rules will work throught that interface.
This option is true by default for kvm-cluster, and false for single and resilient kvm.

A new recipe deploy a python HTTPServer which is used to share files between all vm in the cluster
the url is something like: https:IPv6:port/HASH-PATH/FILES
Access to root https:IPv6:port/ is forbiden.

kvm_run script can now create external disk with specified format (qcow2, raw, etc...)
if kvm external disk is created it will never be replaced, even if user
reduce the number of disk with parameter 'external-disk-number' or change the
'external-disk-format' value. All created disk will be added to qemu run command.

numa option can be used to create [fake] numa nodes and expose them to the guest
OS instance. In this case it is important to know the host cpu topology and where
the instance will be allocated.

cpu-options are additional parameters like sockets=1,cores=5,threads=1
in this case if cpu-count parameter is 1 qemu options will be:
-smp 1,sockets=1,cores=5,threads=1

A new parameter 'use-nat', True by default will specify if we will setup a nat interface.
if 'use-tap' is Truei (false by default), a tap interface will be connected with a new interface in kvm.
if 'use-tap' and 'use-nat' are True, then virtual machine will have two interfaces, one with tap and
one with nat.