linux/balloon: don't allow ballooning down a domain below a reasonable limit

linux/balloon: don't allow ballooning down a domain below a reasonable limit

Reasonable is hard to judge; we don't want to disallow small domains.
But the system needs a reasonable amount of memory to perform its
duties, set up tables, etc. If on the other hand, the admin is able
to set up and boot up correctly a very small domain, there's no point
in forcing it to be larger. We end up with some kind of logarithmic
function, approximated.

Signed-off-by: Kurt Garloff <garloff@suse.de>
Signed-off-by: Jan Beulich <jbeulich@novell.com>