[CORE] Extended IRQs improvments for xvisor-next done.

- The extended IRQs are now always enabled.
- Macros and structures have been renamed to all have a "host_extirq" part in
  common.
- The "vmm_host_extirqs" structure have been renamed "vmm_host_extirq_group".
- A bitmap is used for extended IRQ allocation.
- Error management have been improved.
- A rework to allow using Linux "irq_domain" have been done, but you have to
  use "vmm_host_extirq_to_hwirq" to convert an global IRQ to the domain IRQ.

Moreover, group allocation is dynamic, using a "dlist".
[COMMANDS] Adding extended IRQ command.

A new command, "host extirq stats" have been added to dump extended IRQs.
For now, it is more to debug extended IRQs, but it should evolve quickly to a
better command interface.