support VFIO_IRQ_INFO_MASKABLE

[jlevon]

We have some plumbing for mask/unmask (see #694) but never report VFIO_IRQ_INFO_MASKABLE, so in theory we should never hit that code.

However, it appears that qemu vfio-user client is doing it anyway for INTX, although not for msix

Need to figure out what the proper behaviour should be.

[jlevon]

vfio-pci

1181         if (info.index == VFIO_PCI_INTX_IRQ_INDEX)                               
1182                 info.flags |=                                                    
1183                         (VFIO_IRQ_INFO_MASKABLE | VFIO_IRQ_INFO_AUTOMASKED);     

[jlevon]

we should probably always report INTX as maskable, and actually respect that.

For MSI-X, as need to do #807 and also handle a trigger while masked. I think that looks like:

• setting the pending bit into the PBA on vfu_trigger_irq(msix index)
• accept mask/unmask operations
• on unmask, trigger eventfd for any pending bits

In theory API user could do this too, but seems better handled in the library. And perhaps we need a better MSI-X set up API than we currently have for this.