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

Author: edmondsw

nova/virt/libvirt/driver.py

@@ -278,7 +278,7 @@ def repr_method(self):
 # Mediated devices support
 MIN_LIBVIRT_MDEV_SUPPORT = (3, 4, 0)
 
-# libvirt>=3.10 is required for volume multiattach if qemu<2.10.
+# libvirt>=3.10 is required for volume multiattach unless qemu<2.10.
 # See https://bugzilla.redhat.com/show_bug.cgi?id=1378242
 # for details.
 MIN_LIBVIRT_MULTIATTACH = (3, 10, 0)