caps: Improve error if passed an unknown arch

caps: Improve error if passed an unknown arch

Previously we would have:

"os type 'hvm' & arch 'idontexist' combination is not supported"

Now we get

"No guest options available for arch 'idontexist'"

or if options available but guest OS type not applicable:

"No os type 'xen' available for arch 'x86_64'"