vmm: Enable guest to report free pages through virtio-balloon

Adding a new parameter free_page_reporting=on|off to the balloon device
so that we can enable the corresponding feature from virtio-balloon.

Running a VM with a balloon device where this feature is enabled allows
the guest to report pages that are free from guest's perspective. This
information is used by the VMM to release the corresponding pages on the
host.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

virtio-devices/src/balloon.rs

@@ -392,6 +392,7 @@ impl Balloon {
         id: String,
         size: u64,
         deflate_on_oom: bool,
+        _free_page_reporting: bool,
         seccomp_action: SeccompAction,
         exit_evt: EventFd,
     ) -> io::Result<Self> {