[CORE] vmm_devdrv: Add public fields for IOMMU framework

The IOMMU framework requires "iommu_ops" field in struct vmm_bus
and "iommu_group" in struct vmm_device hence this patch.

This is more of preparatory patch for IOMMU framework.

Signed-off-by: Anup Patel <anup@brainfault.org>