Use multi-vector MSI interrupts in qwx(4).

Use multi-vector MSI interrupts in qwx(4).

With only one vector enabled we need to manually check various rings
to see why an interrupt triggered. This makes debugging difficult at
this early stage of development, so use multiple vectors if possible,
as the Linux driver was designed to do.
The intention is to keep single-vector mode working as well but for
now this mode will lack testing by myself.

Thanks to kettenis@ for adding the required MSI/PCI code on amd64.