doc: update description about security labels on formatdomain.html
doc: update description about security labels on formatdomain.html

This patch adds a brief description about labels for each security
driver.

jfehlig/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,rlaager/libvirt,taget/libvirt,warewolf/libvirt,olafhering/libvirt,crobinso/libvirt,jardasgit/libvirt,datto/libvirt,nertpinx/libvirt,zippy2/libvirt,eskultety/libvirt,shugaoye/libvirt,cbosdo/libvirt,jfehlig/libvirt,cbosdo/libvirt,agx/libvirt,siboulet/libvirt-openvz,jeckersb/libvirt,jfehlig/libvirt,cbosdo/libvirt,libvirt/libvirt,emaste/libvirt,taget/libvirt,novel/fbsd-libvirt,rlaager/libvirt,warewolf/libvirt,shugaoye/libvirt,bjzhang/libvirt,eskultety/libvirt,warewolf/libvirt,bjzhang/libvirt,fabianfreyer/libvirt,iam-TJ/libvirt,zhlcindy/libvirt-1.1.4-maintain,shugaoye/libvirt,datto/libvirt,novel/fbsd-libvirt,siboulet/libvirt-openvz,cbosdo/libvirt,rlaager/libvirt,siboulet/libvirt-openvz,bjzhang/libvirt,zippy2/libvirt,zhlcindy/libvirt-1.1.4-maintain,fabianfreyer/libvirt,nertpinx/libvirt,eskultety/libvirt,siboulet/libvirt-openvz,taget/libvirt,jeckersb/libvirt,fabianfreyer/libvirt,emaste/libvirt,crobinso/libvirt,novel/fbsd-libvirt,nertpinx/libvirt,agx/libvirt,taget/libvirt,fabianfreyer/libvirt,datto/libvirt,trainstack/libvirt,cbosdo/libvirt,trainstack/libvirt,zippy2/libvirt,olafhering/libvirt,warewolf/libvirt,bjzhang/libvirt,eskultety/libvirt,andreabolognani/libvirt,jfehlig/libvirt,rlaager/libvirt,iam-TJ/libvirt,iam-TJ/libvirt,novel/fbsd-libvirt,shugaoye/libvirt,eskultety/libvirt,libvirt/libvirt,emaste/libvirt,trainstack/libvirt,elmarco/libvirt,zhlcindy/libvirt-1.1.4-maintain,olafhering/libvirt,iam-TJ/libvirt,agx/libvirt,elmarco/libvirt,datto/libvirt,jeckersb/libvirt,nertpinx/libvirt,novel/fbsd-libvirt,elmarco/libvirt,zippy2/libvirt,jeckersb/libvirt,libvirt/libvirt,libvirt/libvirt,novel/fbsd-libvirt,emaste/libvirt,VenkatDatta/libvirt,crobinso/libvirt,siboulet/libvirt-openvz,VenkatDatta/libvirt,VenkatDatta/libvirt,warewolf/libvirt,jardasgit/libvirt,emaste/libvirt,datto/libvirt,emaste/libvirt,jeckersb/libvirt,andreabolognani/libvirt,zhlcindy/libvirt-1.1.4-maintain,trainstack/libvirt,trainstack/libvirt,jardasgit/libvirt,elmarco/libvirt,shugaoye/libvirt,zhlcindy/libvirt-1.1.4-maintain,jeckersb/libvirt,andreabolognani/libvirt,jeckersb/libvirt,VenkatDatta/libvirt,emaste/libvirt,jardasgit/libvirt,rlaager/libvirt,olafhering/libvirt,novel/fbsd-libvirt,nertpinx/libvirt,novel/fbsd-libvirt,crobinso/libvirt,bjzhang/libvirt,trainstack/libvirt,andreabolognani/libvirt,elmarco/libvirt,iam-TJ/libvirt,agx/libvirt,iam-TJ/libvirt,trainstack/libvirt,novel/fbsd-libvirt,VenkatDatta/libvirt,agx/libvirt,jardasgit/libvirt,warewolf/libvirt,iam-TJ/libvirt,warewolf/libvirt,taget/libvirt