Drivers: hv: balloon: Make adjustments to the pressure report

The host expects that the pressure report includes the pressure due to the
pages that have been ballooned. Make necessary adjustments to reflect that.
Also, include the free memory information in the pressure report.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>