vfio-pci: Rework MSIX setup/teardown

vfio-pci: Rework MSIX setup/teardown

We try to do lazy initialization of MSIX since we don't actually need
to setup anything until MSIX vectors start getting used.  This leads
to problems if MSIX is enabled, but never used (we can end up trying
to re-enable INTx while it's still enabled).  We also run into
problems trying to expand our reset function to tear down interrupts
as we can then get vector release notifications after we've released
data structures.  By making explicit initialization and teardown we
can avoid both of these problems and behave more similar to bare
metal.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>