x86, msi: Introduce x86_msi.compose_msi_msg call-back

This call-back points to the right function for initializing
the msi_msg structure. The old code for msi_msg generation
was split up into the irq-remapped and the default case.

The irq-remapped case just calls into the specific Intel or
AMD implementation when the device is behind an IOMMU.
Otherwise the default function is called.

Signed-off-by: Joerg Roedel <joro@8bytes.org>
Acked-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>