Introduce MSI and MSI-X support to intrng. This adds a new msi device

Introduce MSI and MSI-X support to intrng. This adds a new msi device
interface with 5 methods to mirror the 5 MSI/MSI-X methods in the pcib
interface. The pcib driver will need to perform a device specific lookup
to find the MSI controller and pass this to intrng as the xref. Intrng
will finally find the controller and have it handle the requested operation.

Obtained from:	ABT Systems Ltd
MFH:		yes
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D5985