core: rename methods in DiskImage

Rename get/setActualSizeFromDiskImage to get/setActualSizeInBytes
to reflect better what the method returns, and rename the
corresponding class member as well.

Change-Id: Iaf26bda8e00c1ae2f12b1decf6e1b4a8d12bed31
Signed-off-by: Alissa Bonas <abonas@redhat.com>
Signed-off-by: Allon Mureinik <amureini@redhat.com>