qemu-option: Introduce default mechanism

This adds qemu_opts_set_defaults, an interface provide default values
for a QemuOpts set. Default options are parsed from a string and then
prepended to the list of existing options, or they serve as the sole
QemuOpts set.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>