QemuOpts: check NULL opts in qemu_opt_get functions

QemuOpts: check NULL opts in qemu_opt_get functions

Some places will call bdrv_create_file(filename, NULL, &local_err), where
opts is NULL. Check NULL in qemu_opt_get and qemu_opt_get_*_del functions,
to avoid extra effort of checking opts before calling them every time.

Signed-off-by: Chunyan Liu <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>