CA-144385 Be more selective about updating vm guest metrics

If "other" has changed but xenstore data/updated is not set, then
do not create a guest metric records if one does not exist already,
and if there is one then update only the "other" map.

This is to fix a bug in which we were populating the guest metrics
record of a cloned VM with network details (IP address) inherited
from the original VM.

Signed-off-by: Thomas Sanders <thomas.sanders@citrix.com>