CA-147159: don't spam the logs for low mem mode event

Although knowing when tapdisk enters in or exits from low memory mode is
a useful piece of information for debugging, the logs get severely
spammed in the presence of many VBDs, increasing the chances that dom0
runs out of disk space.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Chandrika Srinivasan <chandrika.srinivasan@citrix.com>

GitHub: closes #136 on xapi-project/blktap