error_setg_errno() is meant for cases where we got an errno from the OS
that can add useful extra information to an error message. It's
pointless if we pass a constant errno, these cases should use plain
error_setg().

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>