lib: add vprintf

va_report_xfail has the pattern

 char buf[]
 vsnprintf(buf, ...)
 puts(buf)

Before adding another one, replace it with vprintf.

Suggested-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Thomas Huth <thuth@redhat.com>