Don't print device name on failure

Don't print device name on failure

Noticed by mistake (wanted `-l'):
	# vnconfig l
	vnd0
	vnconfig: VNDIOCSET: No such file or directory

Same happens if you try to load a bogus file:
	# vnconfig ./empty
	vnd0
	vnconfig: VNDIOCSET: Input/output error

In both cases, the info on stdout is useless as vnd0 is not used.
Defer printing the device until after the file is set up:
	# ./obj/vnconfig l
	vnconfig: VNDIOCSET: No such file or directory
	# ./obj/vnconfig ./empty
	vnconfig: VNDIOCSET: Input/output error

OK deraadt