passthrough: don't use open coded IO-APIC accesses

passthrough: don't use open coded IO-APIC accesses

This makes the respective functions quite a bit more legible.

Since this requires fiddling with __ioapic_{read,write}_entry()
anyway,
make them and their wrappers have their argument types match those of
__io_apic_{read,write}() (int -> unsigned int).

No functional change intended.

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