Handle difference in xml attribute names in vcloud 5.1 and 5.5 (using the 5.1 api)

Vcloud 5.5 uses a different attribute name for the data that describes the size of the attached disks.

Though the vcloud tools use fog's 5.1 vcloud director api no matter the version of vcloud director (i.e. 5.5), it appears that a few fields have changed.