CA-122467: Fix for setting the right value of memory_actual after vm is started

There is a race condition between vm_start and monitor thread.
The monitor thread runs every 5s and updates the memory changes of the
VMs if any. It does that only if the VM's resident_on value is set to
localhost.
Currently, the memory changes are being seen by monitor thread while
VM is starting but since the start operation has'nt yet set the
resident_on, the monitor thread doesn't do anything.
However, the in-memory cache of monitor thread already has the vm_memory
changes for that VM. This needs to be removed, so that monitor thread
updates it next time when the resident_on value of the VM is set by
start operation.
Signed-off-by: Ravi Pandey <ravip@drall.uk.xensource.com>