Fix format of table "Additional Security Features"

In chapter "40. Hypervisor Selection" of the security-guide, the table
under the heading "Additional Security Features" used an inconsistent
formatting: mix of upper-/lowercase "X", additional whitespace, and in
one case an extra "<address>" tag. Fixed.

Link to the current online version:
http://docs.openstack.org/security-guide/content/ch051_vss-intro.html#ch051_vss-intro-idp401408

Change-Id: I9333ff78b9429a1429cca279c4aa71940f3ec3f5
Merge "Fix format of table "Additional Security Features""