xen: switch extra memory accounting to use pfns

xen: switch extra memory accounting to use pfns

Instead of using physical addresses for accounting of extra memory
areas available for ballooning switch to pfns as this is much less
error prone regarding partial pages.

Reported-by: Roger Pau Monné <roger.pau@citrix.com>
Tested-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
(cherry picked from commit 626d7508664c4bc8e67f496da4387ecd0c410b8c)
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>