update xen/vmware virt drivers not to hit db directly

This change replaces calls to compute api with calls to
conductor api updated with a 'compute_reboot' function.

Fix for bug 1177993

Change-Id: I9aefba95cce29c1486fdfb25008f8da9ece1c19b
Merge "update xen/vmware virt drivers not to hit db directly"