KVM: x86/mmu: replace root_level with cpu_role.base.level

Remove another duplicate field of struct kvm_mmu.  This time it's
the root level for page table walking; the separate field is
always initialized as cpu_role.base.level, so its users can look
up the CPU mode directly instead.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>