iommu/vt-d: Avoid freeing virtual machine domain in free_dmar_iommu()

Virtual machine domains are created by intel_iommu_domain_init() and
should be destroyed by intel_iommu_domain_destroy(). So avoid freeing
virtual machine domain data structure in free_dmar_iommu() when
doamin->iommu_count reaches zero, otherwise it may cause invalid
memory access because the IOMMU framework still holds references
to the domain structure.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>