KVM: x86/mmu: Move individual kvm_mmu initialization into common helper

Move initialization of 'struct kvm_mmu' fields into alloc_mmu_pages() to
consolidate code, and rename the helper to __kvm_mmu_create().

No functional change intended.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200923163314.8181-1-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>