KVM: selftests: Drop the single-underscore ioctl() helpers

KVM: selftests: Drop the single-underscore ioctl() helpers

Drop _kvm_ioctl(), _vm_ioctl(), and _vcpu_ioctl(), as they are no longer
used by anything other than the no-underscores variants (and may have
never been used directly).  The single-underscore variants were never
intended to be a "feature", they were a stopgap of sorts to ease the
conversion to pretty printing ioctl() names when reporting errors.

Opportunistically add a comment explaining when to use __KVM_IOCTL_ERROR()
versus KVM_IOCTL_ERROR().  The single-underscore macros were subtly
ensuring that the name of the ioctl() was printed on error, i.e. it's all
too easy to overlook the fact that using __KVM_IOCTL_ERROR() is
intentional.

Link: https://lore.kernel.org/r/20231108010953.560824-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>