kvm tools: Tune 'kvm balloon' option

This patch changes the option
from:

   kvm balloon [command] [instance name] [amount]

to:

   kvm balloon [inflate/deflate] [size in MiB] [instance name]

Acked-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>