'root_hpa' out of loop in PAE shadow path

The value of 'vcpu->arch.mmu.pae_root' is not modified, so we can update
'root_hpa' out of the loop.

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
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