KVM: nVMX: Rename handle_vm{on,off}() to handle_vmx{on,off}()

Rename the exit handlers for VMXON and VMXOFF to match the instruction
names, the terms "vmon" and "vmoff" are not used anywhere in Intel's
documentation, nor are they used elsehwere in KVM.

Sadly, the exit reasons are exposed to userspace and so cannot be renamed
without breaking userspace. :-(

Fixes: ec378aee ("KVM: nVMX: Implement VMXON and VMXOFF")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220607213604.3346000-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>