KVM/arm64 fixes for 7.1, take #5
- Correctly drop the ITS translation cache reference when it actually
gets invalidated
- Take the SRCU lock for SW page table walks
- Restore POR_EL0 access to host EL0, avoiding POR_EL0 becoming
inaccessible from EL0 after running a guest
- Reassign nested_mmus array behind mmu_lock, ensuring that vcpu init
and MMU notifiers are mutually exclusive
- Correctly handle FEAT_XNX at stage-2