x86: split MSI IRQ chip

x86: split MSI IRQ chip

With the .end() accessor having become optional and noting that
several of the accessors' behavior really depends on the result of
msi_maskable_irq(), the splits the MSI IRQ chip type into two - one
for the maskable ones, and the other for the (MSI only) non-maskable
ones.

At once the implementation of those methods gets moved from io_apic.c
to msi.c.

Signed-off-by: Jan Beulich <jbeulich@suse.com>