QemuOpts: cleanup tmp 'allocated' member from QemuOptsList

QemuOpts: cleanup tmp 'allocated' member from QemuOptsList

Now only qemu_opts_append uses 'allocated' to indicate free memory.
For this function only, we can also let result list's (const char *)
members point to input list's members, only if the input list has
longer lifetime than result list. In current code, that is true.
So, we can remove the 'allocated' member from QemuOptsList definition
to keep code clean.

Signed-off-by: Chunyan Liu <cyliu@suse.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>