iommu/vt-d: Init QI before root entry is allocated

QI needs to be available when we write the root entry into
hardware because flushes might be necessary after this.

Tested-by: ZhenHua Li <zhen-hual@hp.com>
Tested-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>