PCI: hv: Rework MSI handling

Replace the about to vanish iterators and make use of the filtering. Take
the descriptor lock around the iterators.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Michael Kelley <mikelley@microsoft.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/20211206210748.629363944@linutronix.de