irqdomain: Stop using 'host' for domain

It is confusing to see 'host' and 'domain' to be used as 'domain'. Given
this header is all about domains, switch the remaining 'host' uses to
'domain'.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250319092951.37667-5-jirislaby@kernel.org