KVM: MMU: Remove #ifdef CONFIG_X86_64 to support 4 level EPT

Currently EPT level is 4 for both pae and x86_64. The patch remove the #ifdef
for alloc root_hpa and free root_hpa to support EPT.

Signed-off-by: Sheng Yang <sheng.yang@intel.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>