Recalculate partition sizes when the selected disk changes

Don't assume that a valid _model means we want to keep the sizes
already input. If the selected disk changed, reset it and
recalculate.

Change-Id: I799256d4801319af6d48ce9fa7ff24143985798b
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1065823
Signed-off-by: Ryan Barry <rbarry@redhat.com>