QemuOpts: add def_value_str to QemuOptDesc

QemuOpts: add def_value_str to QemuOptDesc

Add def_value_str (default value) to QemuOptDesc, to replace function of the
default value in QEMUOptionParameter.

Improve qemu_opts_get_* functions: if find opt, return opt->str; otherwise,
if desc->def_value_str is set, return desc->def_value_str; otherwise, return
input defval.

Improve qemu_opts_print: if option is set, print opt->str; otherwise, if
desc->def_value_str is set, print it.

Signed-off-by: Dong Xu Wang <wdongxu@linux.vnet.ibm.com>
Signed-off-by: Chunyan Liu <cyliu@suse.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>