Avoid enabling MSI-X if MSI-X is disabled globally.

[shurd]

Summary

It was reported on the community call that with
hw.pci.enable_msix=0, iflib would enable MSI-X on the device and attempt
to use it, which caused issues. Test the sysctl explicitly and do not
enable MSI-X if it's disabled globally.

Test Plan

Have reporter test, test for regressions on physical and
virtual systems.

[cem]

Is this really better than just exposing whatever variable hw.pci.enable_msix corresponds to?

[shurd]

In D12805#266524, @cem wrote:

Is this really better than just exposing whatever variable hw.pci.enable_msix corresponds to?

It's certainly less intrusive. Until I understand exactly why iflib is rewriting PCIM_MSIXCTRL_MSIX_ENABLE in the PCI config register (the comment points to virtualized environments), I want to keep it simple to delete this change if I end up deleting the call to pci_write_config().