x86: Add a test to check effective permissions

Add a test to verify that KVM correctly handles the case where two or
more non-leaf page table entries point at the same table gfn, but with
different parent access permissions.

For example, here is a shared pagetable:
   pgd[]   pud[]        pmd[]            virtual address pointers
                     /->pmd1(u--)->pte1(uw-)->page1 <- ptr1 (u--)
        /->pud1(uw-)--->pmd2(uw-)->pte2(uw-)->page2 <- ptr2 (uw-)
   pgd-|           (shared pmd[] as above)
        \->pud2(u--)--->pmd1(u--)->pte1(uw-)->page1 <- ptr3 (u--)
                     \->pmd2(uw-)->pte2(uw-)->page2 <- ptr4 (u--)
  pud1 and pud2 point to the same pmd table

The test is useful when TDP is not enabled.

Co-Developed-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Message-Id: <20210605174901.157556-1-jiangshanlai@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>