KVM: x86: Use more verbose names for mem encrypt kvm_x86_ops hooks

Use slightly more verbose names for the so called "memory encrypt",
a.k.a. "mem enc", kvm_x86_ops hooks to bridge the gap between the current
super short kvm_x86_ops names and SVM's more verbose, but non-conforming
names.  This is a step toward using kvm-x86-ops.h with KVM_X86_CVM_OP()
to fill svm_x86_ops.

Opportunistically rename mem_enc_op() to mem_enc_ioctl() to better
reflect its true nature, as it really is a full fledged ioctl() of its
own.  Ideally, the hook would be named confidential_vm_ioctl() or so, as
the ioctl() is a gateway to more than just memory encryption, and because
its underlying purpose to support Confidential VMs, which can be provided
without memory encryption, e.g. if the TCB of the guest includes the host
kernel but not host userspace, or by isolation in hardware without
encrypting memory.  But, diverging from KVM_MEMORY_ENCRYPT_OP even
further is undeseriable, and short of creating alises for all related
ioctl()s, which introduces a different flavor of divergence, KVM is stuck
with the nomenclature.

Defer renaming SVM's functions to a future commit as there are additional
changes needed to make SVM fully conforming and to match reality (looking
at you, svm_vm_copy_asid_from()).

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220128005208.4008533-20-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>