x86/MSI-X: be more careful during teardown

x86/MSI-X: be more careful during teardown

When a device gets detached from a guest, pciback will clear its
command register, thus disabling both memory and I/O decoding. The
disabled memory decoding, however, has an effect on the MSI-X table
accesses the hypervisor does: These won't have the intended effect
anymore. Even worse, for PCIe devices (but not SR-IOV virtual
functions) such accesses may (will?) be treated as Unsupported
Requests, causing respective errors to be surfaced, potentially in the
form of NMIs that may be fatal to the hypervisor or Dom0 is different
ways. Hence rather than carrying out these accesses, we should avoid
them where we can, and use alternative (e.g. PCI config space based)
mechanisms to achieve at least the same effect.

At this time it continues to be unclear whether this is fixing an
actual bug or is rather just working around bogus (but apparently
common) system behavior.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>