If x2apic is not available, hyperv-iommu skips remapping
irqs. This breaks root partition which always needs irqs
remapped.

Fix this by allowing irq remapping regardless of x2apic,
and change hyperv_enable_irq_remapping() to return
IRQ_REMAP_XAPIC_MODE in case x2apic is missing.

Tested with root and non-root hyperv partitions.
Signed-off-by: Nuno Das Neves <nunodasneves@linux.microsoft.com>
Reviewed-by: Tianyu Lan <Tianyu.Lan@microsoft.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/1668715899-8971-1-git-send-email-nunodasneves@linux.microsoft.comSigned-off-by: Wei Liu <wei.liu@kernel.org>