vmm: keep memory decode enabled through MSI-X teardown

[nick_spun.io]

Summary

ppt_unassign_device() and ppt_reset_device() cleared Memory Space
Enable before tearing down MSI-X. Masking a vector writes the MSI-X
table, which lives in a device BAR, so once decode is disabled the
write is an Unsupported Request. On a live guest (msix_alloc > 0)
the UR is reported through APEI as a fatal hardware error and the
host panics.

Tear down MSI/MSI-X with decode still on, then gate decode and DMA
before the reset.

See PCI Express Base Specification Revision 5.0 section 7.5.1.1.3
(Command register, Memory Space Enable)

[nick_spun.io]

handle PCI_QUIRK_MSI_INTX_BUG

[wanpengqian_gmail.com]

Makes sense: ppt_teardown_msix_intr() -> bus_teardown_intr() -> pci_mask_msix() writes the vector control dword in the table BAR, so with decode off that is a UR. One suggestion: the old ordering also stopped DMA before the interrupt teardown (the device could still be bus-mastering while its guest mappings go away), and the new one leaves BUSMASTEREN on until afterwards. Clearing BUSMASTEREN together with INTxDIS first, and only deferring PORTEN/MEMEN until after the MSI-X teardown, keeps both properties. (MSI teardown doesn't touch BARs; only MSI-X does.) We carry the same ordering in keelOS's ppt (stable/15) and will pick this up.