x86/intel-iommu: make edu_dev global

We'll need to access the edu_dev struct from the ISR, so make it
global.

Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>