qtest: implement QTEST_STOP

qtest: implement QTEST_STOP

It is quite difficult to debug qtest test cases without extra wrapper
scripts for QEMU or similar.  This patch adds a simple environment
variable-based trigger that sends a STOP signal to the QEMU instance
under test, before attempting to connect to its QMP session.

This will block execution of the testcase and give time to attach a
debugger to the stopped QEMU process.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>