xen/virtio: fix balloon drivers to not mark pages as WIRED

Prevent the Xen and VirtIO balloon drivers from marking pages as
wired. This prevents them from increasing the system wired page count,
which can lead to mlock failing because of hitting the limit in
vm.max_wired.

In the Xen case make sure pages are zeroed before giving them back to
the hypervisor, or else we might be leaking data. Also remove the
balloon_{append/retrieve} and link pages directly into the
ballooned_pages queue using the plinks.q field in the page struct.

Sponsored by: Citrix Systems R&D
Reviewed by: kib, bryanv
Approved by: gibbs

dev/virtio/balloon/virtio_balloon.c:
 - Don't allocate pages with VM_ALLOC_WIRED.

dev/xen/balloon/balloon.c:
 - Don't allocate pages with VM_ALLOC_WIRED.
 - Make sure pages are zeroed before giving them back to the
   hypervisor.
 - Remove the balloon_entry struct and the balloon_{append/retrieve}
   functions and use the page plinks.q entry to link the pages
   directly into the ballooned_pages queue.
Mark send-pr info page as an obsolete file