Ads support to the xhci pci attachment to use MSI-X interrupts when
available. As with MSI interrupts these can be disabled by setting
hw.usb.xhci.msix to 0 in the loader.

MSI-X interrupts are needed on some hardware, for example the Cavium
ThunderX only supports them, and with this we don't fall back to polling.

PR: 204378
Reviewed by: hselasky, jhb
MFC after: 1 week (after r292669)
Sponsored by: ABT Systems Ltd
Differential Revision: https://reviews.freebsd.org/D4698