Implement multivector MSI IRQ setup.  This allows to set up and use multiple
MSI IRQs per device.

[bhelgaas: changelog, use -EINVAL instead of -ENOSYS]
Signed-off-by: NLucas Stach <l.stach@pengutronix.de>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Acked-by: NPratyush Anand <pratyush.anand@gmail.com>