KVM: x86/mmu: Consolidate shadow page allocation and initialization

Consolidate kvm_mmu_alloc_page() and kvm_mmu_alloc_shadow_page() under
the latter so that all shadow page allocation and initialization happens
in one place.

No functional change intended.

Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20220516232138.1783324-8-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>