Introduce QString unit-tests

This suite contains tests to assure that QString API works as expected.

To execute it you should have check installed and build QEMU with
check support enabled (--enable-check-utests) and then run:

$ ./check-qstring

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
