vfio: fix build with Linux < 4.0

vfio: fix build with Linux < 4.0

Since the older kernel version do not implement the device request
interface for vfio, so when build on the kernel < v4.0.0, which is
the version begin to add the device request interface, it will
throw the error to show “VFIO_PCI_REQ_IRQ_INDEX” is undeclared.
This patch aim to fix this compile issue by add the macro
“HAVE_VFIO_DEV_REQ_INTERFACE” after checking the kernel version.

Fixes: 0eb8a1c4c786 ("vfio: add request notifier interrupt")
Fixes: c115fd000c32 ("vfio: handle hotplug request notifier")

Signed-off-by: Jeff Guo <jia.guo@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>