Add volume filesystem total attribute to mgmt instance get

Currently the instance get call returns the 'used' filesystem stats and the volume size but there is no easy way to confirm that volume size (which Cinder gives) matches what the nova server filesystem recognizes.

The guestagent filesystem stats already gets the total and free bytes in order to calculate 'used' stat. This change would return total in GB with 'used' to trove.

Adding to non-mgmt instance.get call would be confusing to users with the cinder size attribute already being returned.