libqtest: Wait for the right child PID after killing QEMU

When running "make check" with gcov enabled, we get the following
message:

   hw/tmp105.gcda:cannot open data file, assuming not executed

The problem happens because:

 * tmp105-test exits before QEMU exits, because waitpid() at
   qtest_quit() fails;
 * waitpid() fails because there's another process already
   waiting for the QEMU process;
 * The process that is already waiting for QEMU is the child created by
   qtest_init() to run system();
 * qtest_quit() is incorrectly waiting for the QEMU PID directly instead
   of the child created by qtest_init().

This fixes the problem by sending SIGTERM to QEMU, but waiting for the
child process created by qtest_init() (that exits immediately after QEMU
exits).

Reported-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
