KVM: x86/mmu: Refactor yield safe root iterator

Refactor the yield safe TDP MMU root iterator to be more amenable to
changes in future commits which will allow it to be used under the MMU
lock in read mode. Currently the iterator requires a complicated dance
between the helper functions and different parts of the for loop which
makes it hard to reason about. Moving all the logic into a single function
simplifies the iterator substantially.

Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20210401233736.638171-6-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>