iommu/vt-d: Replace iommu_bmp with a refcount

This replaces the dmar_domain->iommu_bmp with a similar
reference count array. This allows us to keep track of how
many devices behind each iommu are attached to the domain.

This is necessary for further simplifications and
optimizations to the iommu<->domain attachment code.

Signed-off-by: Joerg Roedel <jroedel@suse.de>