Configuration: Allow passing of additional QEMU parameters

[mcastelino]

Today the configuration.toml allows passing of additional kernel command line parameters

kernel_params = ""

It would be useful for power users to pass additional (for now) parameters to QEMU to customize its behavior. This would be specially useful for features that may not make sense to be exposed as hypervisor knobs.

Any option that is generally useful should be exposed as a knob. But having the ability to quickly add an option would be useful for rapid customization without code change.

/cc @grahamwhaley