check space against all disks selected instead of just one

Previously, if the Root disk was used for HostVG as well, the
disk space check would only look at the Root disk for total space
and would ignore all other disks specified for HostVG.

rhbz#868776

Change-Id: I9fe745fb82052ce25cf24527c69592a67fa5b035
Signed-off-by: Mike Burns <mburns@redhat.com>