Rejig supported arch stuff to avoid incorrect deletions
    
    The way this was done, when we ran `all` on a ppc host it wiped
    all the x86_64 and i686 virt-install images, and when we ran
    `all` on an x86_64 host it wiped all the ppc64 virt-install
    images. Doing it this way avoids that.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>