kvm: x86/mmu: Support MMIO in the TDP MMU

kvm: x86/mmu: Support MMIO in the TDP MMU

In order to support MMIO, KVM must be able to walk the TDP paging
structures to find mappings for a given GFN. Support this walk for
the TDP MMU.

Tested by running kvm-unit-tests and KVM selftests on an Intel Haswell
machine. This series introduced no new failures.

This series can be viewed in Gerrit at:
	https://linux-review.googlesource.com/c/virt/kvm/kvm/+/2538

v2: Thanks to Dan Carpenter and kernel test robot for finding that root
was used uninitialized in get_mmio_spte.

Signed-off-by: Ben Gardon <[email protected]>
Reported-by: kernel test robot <[email protected]>
Reported-by: Dan Carpenter <[email protected]>
Message-Id: <20201014182700.2888246[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>