qtest: Don't segfault with invalid -qtest option

This prints an error message, instead of core dump, when "-qtest"
option value is invalid, e.g.:

    $ ./x86_64-softmmu/qemu-system-x86_64 -qtest unknown
        qemu-system-x86_64: Failed to initialize device for qtest:
        "unknown"

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
