disable guest-provided stats on "info balloon" command

The addition of memory stats reporting to the virtio balloon causes
the 'info balloon' command to become asynchronous.  This is a regression
because in some cases it can hang the user monitor.

This is an alternative to Adam Litke's patch. Adam's patch disabled the
corresponding (guest-visible) virtio feature bit, causing issues for migration.
Original discussion is available at:
http://marc.info/?l=qemu-devel&m=128448124328314&w=2



Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Acked-by: Adam Litke <agl@us.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
(cherry picked from commit 07b0403d)