Allow greater customization of virtual hardware

[cryptoknight]

Transient adds several hardware devices by default to its QEMU configurations including virtio-scsi-pci (along with scsi-hd's), virtio-net-pci, and a serial device. Each of these should have a config file option to disable their inclusion so that a user can specify their own preferred device types and options to QEMU directly. Notes:

• The (undocumented) no_virtio_scsi option exists already, but id= fields should probably be added to the -drive options it generates so user-added devices can be bound to the proper instance.
• The (undocumented) ssh_net_driver option also exists, but something like no_virtio_net would be more flexible and consistent with the above.
• -serial stdio is currently added to every machine with SSH enabled, even if ssh_with_serial is disabled, which makes it inaccessible to the user. This behavior should either be changed by default or a no_serial option added.