Fix pci-passthru MSI issues with OpenBSD guests.

[grehan]

Summary

• Return 2 x 16-bit registers in the correct byte order for a 4-byte read that spands 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.

PR 245392

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

Test Plan

Set up an OpenBSD guest with a passthru device.

[jhb]

I think this is fine. The other design option would be to have pci_emul_capwrite to always require the explicit offset and id and move the while loop out into a helper function (pci_emul_lookup_cap or some such) that pci_cfgrw would use.