See comment #4 in bug 247457.
create an external kickstart distribution for rhel 5.
Create two activation keys, one for virt host, and one for virt host platform.
Create a kickstart profile using the rhel 5 distribution and the two activation keys
kickstart a system with this profile, and note that it only gets virt host platform.
QA Verified -- after creating a kickstart profile with two activation keys --
one pointing to virt, the other to virt platform, i kickstarted a system. It
resulted in a RHEL5 system with virt platform entitlements:
fjs-0-09.rhndev.redhat.com Red Hat Enterprise Linux (v. 5 for 32-bit x86)
Management, Virtualization Platform