QemuOpt: add unit tests

QemuOpt: add unit tests

Cover basic aspects and API usage for QemuOpt. The current implementation
covers the API's planned to be changed by Chunyan Liu in his QEMUOptionParameter
replacement/cleanup job.

Other APIs should be covered in future improvements.

[Squashing in a small fix "QemuOpt: use qemu_find_opts_err() to avoid
output on stderr in tests".

qemu_find_opts() calls error_report() instead of propagating the Error
object.  It is undesirable to clutter test case output with error
messages from a passing test.

Use qemu_find_opts_err() to avoid the output on stderr.
--Stefan]

Signed-off-by: Leandro Dorileo <l@dorileo.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>