vfio-pci: Clang cleanup

vfio-pci: Clang cleanup

Blue Swirl reports that Clang doesn't like the structure we define to
avoid dynamic allocation for a number of calls to VFIO_DEVICE_SET_IRQS.
Adding an element after a variable sized type is a GNU extension.
Switch back to dynamic allocation, which really isn't a problem since
this is only done on interrupt setup changes.

Cc: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>