vfio: rework vfio_irq_set payload setting

struct vfio_irq_set from the kernel headers contains a variable sized
array to hold a payload. The vfio_irq_eventfd struct puts the "fd"
member right after this, hoping it to automatically fit in the payload slot.
But having a variable sized type not at the end of a struct is a GNU C
extension, so clang will refuse to compile this.

Solve this by somewhat doing the compiler's job and place the payload
manually at the end of the structure.

Reviewed-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>