policy: Fix balloon policy syntax and semantics

This patch fixes two issues in the current balloon policy:
1) In shrink_guest, guest_free_percent should be
   min_guest_free_percent.  This was causing a runtime error for users.
2) When calculating balloon_min, we want to choose whichever number is
   higher between guest.balloon_min and the calculated value.
   Otherwise, guest.balloon_min cannot act as a floor value as it is
   intended to.

Change-Id: I8fba77bb5c42b29ab479752e335cbc11ed0a298b
Signed-off-by: Adam Litke <agl@us.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/19416
Reviewed-by: Saggi Mizrahi <smizrahi@redhat.com>
Reviewed-by: Martin Sivák <msivak@redhat.com>