x86: Virtualize IOAPIC redir table for interrupt remapping support

x86: Virtualize IOAPIC redir table for interrupt remapping support

Handover the IOAPIC on hypervisor setup by first masking all pins, then
requesting interrupt remapping from VT-d and finally reprogramming them
according to the index that VT-d reported. If vtd_map_interrupt returns
-ENOSYS, unconditionally right now due to a missing implementation,
later on when running in QEMU, we continue to write the redirection
table entry unmodified.

As we may lose edge-triggered interrupts while they are makes, we inject
them unconditionally into the target CPU. This may cause one spurious
interrupt per handover to/from the hypervisor, but Linux can deal with
that.

Whenever a cell is added or removed, we rewrite all root cell IOAPIC
redir table entries in order to validate them regarding the target CPU
masks.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>