Improve the build to:
- Encapsulate the qemu configuration.
- Validate the chosen hypervisor.
- Display hypervisor details for `make help`.
This lays the groundwork to build configurations for multiple
hypervisors.
Notes:
- Variables that are hypervisor specific have now been suffixed with `_QEMU`.
- `make install` now installs `configuration-qemu.toml`.
To maintain compatibility with existing installations, a symbolic link
to this file called `configuration.toml` is created.
Signed-off-by: James O. D. Hunt <james.o.hunt@intel.com>
