xen/balloon: don't use PV mode extra memory for zone device allocations

When running as a Xen PV guest use the extra memory (memory which isn't
allocated for the guest at boot time) only for ballooning purposes and
not for zone device allocations. This will remove some code without any
lack of functionality.

While at it move some code to get rid of another #ifdef.

Remove a comment which is stale since some time now.

Signed-off-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20220407093857.1485-1-jgross@suse.com
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>