useful-image-properties: Update note on machine types
useful-image-properties: Update note on machine types

For ARMv7 and AArch64, Nova uses the 'virt' machine type as the default.
Update the docs to reflect that.

Refer to this commit in Nova:

    https://opendev.org/openstack/nova/commit/e155baefb0
    (libvirt: Use 'virt' as the default machine type for ARMv7,
    2018-09-14)

Change-Id: I13793dded976fdf872ca0ea779a7f6cbee98118f
Signed-off-by: Kashyap Chamarthy <005b99b234026d47e167244b248f0ff319fdaf16@redhat.com>

openstack/glance,openstack/glance,openstack/glance