tests/q35-test: push down qtest_start / qtest_end to test case(s)

tests/q35-test: push down qtest_start / qtest_end to test case(s)

A test program can start up QEMU several times, with different command
lines. For such cases, qtest_start() and qtest_end() are called from
within the individual test functions. Examples: "virtio-console-test.c",
"numa-test.c", and many others.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>