Add machine version in kvm runtime file

kvm -M ? returns the supported machines (e.g. pc-1.1).
Add _GetDefaultMachineVersion() function to get the default value.

Upon kvm runtime file creation (this is in _GenerateKVMRuntime() invoked
only in StartInstance()) append this info in kvm_cmd. During
live migration the -incoming kvm process is started based on this file.

In case of different KVM versions between source and target nodes
there is a possibility (e.g. due to a kvm bug) for migration to fail silently.

This patch forces the target node to emulate the same machine version
used by running process. If KVM on target node does not support it
the -incoming kvm process will crash and migration will be aborted.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Reviewed-by: Guido Trotter <ultrotter@google.com>