hw/vfio: Add CONFIG switches for calxeda-xgmac and amd-xgbe

Both devices seem to be specific to the ARM platform. It's confusing
for the users if they show up on other target architectures, too
(e.g. when the user runs QEMU with "-device ?" to get a list of
supported devices). Thus let's introduce proper configuration switches
so that the devices are only compiled and included when they are
really required.

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