webadmin: Display size of ISO and floppy files in human readable format

Up until now DiskSizeRenderer which is used to render image/disk/file
sizes had 2 limitations:
1. Size appeared only in GB.
2. Size smaller than 1GB appeared as '<1GB'

From now on, it also supports widely adopted Human Readable Format.

This patch uses new capability when rendering size of ISO files and
floppies under Storage ->ISO Storage Domain -> Images.

Change-Id: I95961fde2256a44b8474d8f41562bc0e33b0ad4a
Bug-Url: https://bugzilla.redhat.com/1005889
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>