Change the column counter_volume to Float

counter_volume was previously an Integer (32 bits value in SQL
implementations as MySQL or PostgreSQL) and limited in values from
-2147483648 to 2147483647, very small for things like disk I/O bytes,
network I/O bytes, image size, object size, volume size, …
Furthermore one might want to store some real numbers (ℝ) (temperatures,
power comsumption, …)

Change-Id: Ifdb8e8e84e8e01df267d71bc72cdc3f56bb2f695
Fixes: bug #1158276