KVM: x86/mmu: Rename max_page_level to max_huge_page_level

Rename max_page_level to explicitly call out that it tracks the max huge
page level so as to avoid confusion when a future patch moves the max
TDP level, i.e. max root level, into the MMU and kvm_configure_mmu().

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200716034122.5998-9-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>