Revert "Summary: Use hypervisor as clock source for system vms"

removing due to CLOUDSTACK-2872, ubuntu 12.04's libvirt isn't new enough

This reverts commit c7ebcb4e8c287ae88d2f6e6d71cc536612a5e635.
Squeak.

Signed-off-by: Prasanna Santhanam <tsp@apache.org>
(cherry picked from commit 6c1d9170ac00ec48fef1b4c1e2e11b074ff75e91)
CLOUDSTACK-4125: [Upgrade] after upgrade 3.0.7 GA -> 4.2 deploy guest VM
fail.