iommu/vt-d: Introduce helper functions to improve code readability

Introduce domain_type_is_vm() and domain_type_is_vm_or_si() to improve
code readability.

Also kill useless macro DOMAIN_FLAG_P2P_MULTIPLE_DEVICES.

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