transport-pci: Add MSI support

[Mani-Sadhasivam]

MSI is the predecessor of MSI-X that allows PCIe devices to send interrupts
to the host. Compared to MSI-X, MSI supports only a maximum of 32 vectors
per PCIe function. But MSI has been widely supported by the PCIe devices
requiring fewer interrupts such as Modems, WLAN cards etc...

Currently, Virtio spec only documents MSI-X and INTX interrupt mechanisms
for the PCI transport. So if a Virtio device based on PCI transport
supports only MSI, then the driver on the guest will only use INTX for
receiving the interrupts. This is sub-optimal and affects the
performance of the device. With MSI, the device can use one vector
per queue (max of 32 vectors) thus avoiding the overhead associated with a
shared INTX vector.

Fix: https://lore.kernel.org/virtio-comment/[email protected]/