KVM: selftests: Make vm_ioctl() a wrapper to pretty print ioctl name

Make vm_ioctl() a macro wrapper and print the _name_ of the ioctl on
failure instead of the number.

Deliberately do not use __stringify(), as that will expand the ioctl all
the way down to its numerical sequence.  Again the intent is to print the
name of the macro.

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