msix_notify() not working.

[Douglas Su]

I implement a simple PCIe device based on hw/misc/edu.c. I want to use MSI-X 
instead of MSI interrupt mechanism, and replace msi_ functions with msix_ 
counterparts.

In my device driver, I can get the correct interrupt vector number by invoking 
`pci_alloc_irq_vectors()` according to this doc in Kernel source: 
https://github.com/torvalds/linux/blob/master/Documentation/PCI/msi-howto.rst

However, when I writing register to trigger an interrupt in driver, nothing 
happens. 

The logical is simple, I write a command to BAR register, and underlaying a 
`msix_notify()` function is invoked to feed back an interrupt. Also, I do see 
`msix_notify()` is called in GDB.

So, anyone can give me a clue on how to properly send msix interrupt to up