Add a balloon device to all kvm instances

This can be used to resize their memory (for now only manually, as
ganeti doesn't do the job yet)

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>

lib/hypervisor/hv_kvm.py

@@ -962,6 +962,7 @@ class KVMHypervisor(hv_base.BaseHypervisor):
     kvm_cmd.extend(["-m", instance.beparams[constants.BE_MAXMEM]])
     kvm_cmd.extend(["-smp", instance.beparams[constants.BE_VCPUS]])
     kvm_cmd.extend(["-pidfile", pidfile])
+    kvm_cmd.extend(["-balloon", "virtio"])
     kvm_cmd.extend(["-daemonize"])
     if not instance.hvparams[constants.HV_ACPI]:
       kvm_cmd.extend(["-no-acpi"])