Export qemu_opt_find for qcow2 driver using it.
After replacing QEMUOptionParameter with QemuOpts, qcow2 driver will
use qemu_opt_find to judge if an option is explicitly set, to replace
the usage of .assigned in QEMUOptionParameter.

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