virttest/qemu_vm:Fix unable to add nodefaults qemu command line

has_option(help_text, "use_nodefaults") will always return false because
qemu help_text just has 'nodefaults' line, instead of 'use_nodefaults'.
So -nodefaults will never be used as qemu command line.

This patch is to avoid this error situation

Signed-off-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>