tests: Expose regression in QemuOpts visitor

Commit 15c2f669e broke the ability of the QemuOpts visitor to
flag extra input parameters, but the regression went unnoticed
because of missing testsuite coverage.  Add a test to cover this;
take the approach already used in 9cb8ef3 of adding a test that
passes (to avoid breaking bisection) but marks with BUG the
behavior that we don't like, so that the actual impact of the
fix in a later patch is easier to see.

CC: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Message-Id: <20170322144525.18964-2-eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
