CP-4465 contribution: reduce logging of low-value messages
 * log less in block_device_io.action_writedelta
 * shorten a common log line in xapi_session.ml

Signed-off-by: Thomas Sanders <thomas.sanders@citrix.com>
Signed-off-by: Jerome Maloberti <jerome.maloberti@citrix.com>
Merge pull request #1200 from jeromemaloberti/CP-4465-clr

CP-4465 contribution: reduce logging of low-value messages