Maintain backwards compatibility in kvm runtime

Introduce _GenerateKVMBlockDevicesOptions()

In case hotplug is enabled per instance then the runtime file
contains one more field: block_devices. In this case kvm_cmd
is extended with block device options during _ExecuteKVMRuntime().

If no hotplug is enabled then runtime file has block_devices info
encapsulated in kvm_cmd and block device options are exteded
during _GenerateKVMRuntime.

Signed-off-by: Dimitris Aragriorgs <dimara@grnet.gr>