Revision 361442

Fix pci-passthru MSI issues with OpenBSD guests

- Return 2 x 16-bit registers in the correct byte order
 for a 4-byte read that spans the CMD/STATUS register.
  This reversal was hiding the capabilities-list, which prevented
 the MSI capability from being found for XHCI passthru.

- Reorganize MSI/MSI-x config writes so that a 4-byte write at the
 capability offset would have the read-only portion skipped.
  This prevented MSI interrupts from being enabled.

 Reported and extensively tested by Anatoli (me at anatoli dot ws)

PR:	245392
Reported by:	Anatoli (me at anatoli dot ws)
Reviewed by:	jhb (bhyve)
Approved by:	jhb, bz (mentor)
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D24951