KVM: x86: Skip pae_root shadow allocation if tdp enabled

Considering the fact that the pae_root shadow is not needed when
tdp is in use, skip the pae_root shadow page allocation to allow
mmu creation even not being able to obtain memory from DMA32
zone when particular cgroup cpuset.mems or mempolicy control is
applied.

Signed-off-by: Liang Chen <liangchen.linux@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>