KVM: MMU: Inherit a shadow page's guest level count from vcpu setup

Instead of "calculating" it on every shadow page allocation, set it once
when switching modes, and copy it when allocating pages.

This doesn't buy us much, but sets up the stage for inheriting more
information related to the mmu setup.

Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -244,6 +244,7 @@ struct kvm_mmu {
 	hpa_t root_hpa;
 	int root_level;
 	int shadow_root_level;
+	union kvm_mmu_page_role base_role;
 
 	u64 *pae_root;
 };