kvm tools: MSI-X fixes

Several fixes in this patch:

* Don't ignore function level and per-vector masking. We're not
supposed to signal when masked and not doing so will improve
performance a bit (in addition to behaving correctly).

* Implement the missing PBA array. 'lspci -vv' will now show the correct
output:
	Capabilities: [40] MSI-X: Enable+ Count=5 Masked-
		Vector table: BAR=1 offset=00000000
		PBA: BAR=3 offset=00000000

* Checking whether MSI-X is enabled or not is done by probing the
corresponding bit within the PCI header instead of trying to track
the status ourselves.

* Fallback to INTx if MSI-X isn't enabled.

* Use correct max size for MSI-X table.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>