Abstract the common operations from account_shadowed() and
unaccount_shadowed(), then introduce kvm_mmu_gfn_disallow_lpage()
and kvm_mmu_gfn_allow_lpage()

These two functions will be used by page tracking in the later patch
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

CVE-2018-12207

(cherry picked from commit 547ffaed)
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>