Use is_virtual instead of is_virtual_packstack

At least on Virtualbox is_virtual_packstack is not working.

$ grep hypervisor /proc/cpuinfo > /dev/null && echo true || echo false
false

Change-Id: I8a1fec615de3eb774950f75e38dfbf334bd81ba2
Merge "Use is_virtual instead of is_virtual_packstack"