IRQ: u16 is too narrow for an event channel number

IRQ: u16 is too narrow for an event channel number

FIFO event channels allow ports up to 2^17, so we need to use a wider
field in struct pirq. Move "masked" such that it may share the 8-byte
slot with struct arch_pirq on 64-bit arches, rather than leaving a
7-byte hole in all cases.

Take the opportunity and also add a comment regarding "arch" placement
within the structure.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>