fix multiple pci-attach/pci-detach by removing meaningless irq_index

fix multiple pci-attach/pci-detach by removing meaningless irq_index

For HVM guest, we can't do pci-attach/pci-detach for >64 times due to
the meaningless checking in pci_register_device(). This patches
removes it.

With the patches, I [Dexuan] made a test of 300 times of attach/detach
and everything works well.

Signed-off-by: Dexuan Cui <dexuan.cui@intel.com>
(cherry picked from commit 990348aa4d6b55d2d2d1aa60b33385cb77c78052)