Add configurable option for vhost

[cmainas]

A straightforward way to improve the network performance of Qemu (and other future supported monitors) is to enable vhost in the respective cli option for network. However, vhost comes with some security implications, since a bigger chunk of the host kernel interacts with the sandbox. For that reason, instead of enabling vhost by default, we can set it as a configuration option in urunc's config (under the monitor section). In that way, the monitor code should simply check if vhost is enabled and specify the network cli options for the monitor accordingly.