kvm tools: Add virtio-balloon device

kvm tools: Add virtio-balloon device

From the virtio spec:

The virtio memory balloon device is a primitive device for managing guest
memory: the device asks for a certain amount of memory, and the guest supplies
it (or withdraws it, if the device has more than it asks for). This allows the
guest to adapt to changes in allowance of underlying physical memory.

To activate the virtio-balloon device run kvm tools with the '--balloon'
command line parameter.

Current implementation listens for two signals:

 - SIGKVMADDMEM: Adds 1M to the balloon driver (inflate). This will decrease
available memory within the guest.
 - SIGKVMDELMEM: Remove 1M from the balloon driver (deflate). This will
increase available memory within the guest.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>