pci: introduce a bus master container

96a8821d ("virtio: unbreak virtio-pci with IOMMU after caching ring
translations") tries to make IOMMU works with virtio memory region
cache, but it requires IOMMU to be created before any virtio
devices. This is sub optimal, fixing this by introduce a bus master
container to make sure address space can be initialized during device
registering, and then we can safely set alias and make
bus_master_enable_region as its subregion during bus master
initialization.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>