nova can nolonger disabel the mem ballon device

[sean mooney]

https://docs.openstack.org/nova/latest/configuration/config.html#libvirt.mem_stats_period_seconds

setting the mem_stats_period_seconds value to 0 is documented as disabling the memory ballon

in practice it disable the generation of the generate of the

 <memballoon model='virtio'/> element.

in older version of libvirt that had the effect of disabling the mem ballon

in libvirt 0.8.4 libvrt gaind the ability to disable the mem ballon explicitly by setting

<memballoon model='none'/>

and at some later point libvirt started adding a mem ballon deivece by default if one is not present in the XML

while the exact version of libvirt that change was made in is not known the net effect is

[libvirt]
mem_stats_period_seconds=0 no longer disbales the memory balloon as documented.

mem_stats_period_seconds

    Type:

        integer
    Default:

        10

    A number of seconds to memory usage statistics period. Zero or negative value mean to disable memory usage statistics.

disabling the memory balloon and statistics is important for minimising the overhead of tracking that memory usage in realtime vms

since nova does not currently set

autodeflate the mem balloon in nova had no other use except metric reporting so when we set
mem_stats_period_seconds=0 we should explicitly disable the device in the XML to restore the old behavior.

a future enhancement could be to set autodeflate=on when the device is present.

this bug also interact with sev.

sev requires all virtio devices to have the iommu=on driver option set
if you disabel mem stats today and libvirt adds a mem-ballon automatically it will not have this element set and that can cause issues with some feature like sev.