comment correction for libvirt multiattach
comment correction for libvirt multiattach

The libvirt driver had a significant error in a comment, incorrectly
describing qemu/libvirt version interaction for volume multiattach.
This corrects that, bringing it in line with the implementation [1].

[1] https://github.com/openstack/nova/blob/bea1b971789e2ca1778a1285fcfd7704e22b1d6c/nova/virt/libvirt/driver.py#L578-L589

Change-Id: I3faade7cb769b3abee284900582542bb1f28897a

openstack/nova,openstack/nova,openstack/nova,gooddata/openstack-nova,mikalstill/nova,gooddata/openstack-nova,rahulunair/nova,gooddata/openstack-nova,klmitch/nova,rahulunair/nova,mahak/nova,rahulunair/nova,klmitch/nova,mikalstill/nova,gooddata/openstack-nova,klmitch/nova,mahak/nova,mikalstill/nova,klmitch/nova,mahak/nova