tests/qtest/migration-test: Fix the check for a successful run of analyze-migration.py

tests/qtest/migration-test: Fix the check for a successful run of analyze-migration.py

If analyze-migration.py cannot be run or crashes, the error is currently
ignored since the code only checks for nonzero values in case the child
exited properly. For example, if you run the test with a non-existing
Python interpreter, it still succeeds:

 $ PYTHON=wrongpython QTEST_QEMU_BINARY=./qemu-system-x86_64 tests/qtest/migration-test
 ...
 # Running /x86_64/migration/analyze-script
 # Using machine type: pc-q35-9.1
 # starting QEMU: exec ./qemu-system-x86_64 -qtest unix:/tmp/qtest-417639.sock -qtest-log /dev/null -chardev socket,path=/tmp/qtest-417639.qmp,id=char0 -mon chardev=char0,mode=control -display none -audio none -accel kvm -accel tcg -machine pc-q35-9.1, -name source,debug-threads=on -m 150M -serial file:/tmp/migration-test-XPLUN2/src_serial -drive if=none,id=d0,file=/tmp/migration-test-XPLUN2/bootsect,format=raw -device ide-hd,drive=d0,secs=1,cyls=1,heads=1   -uuid 11111111-1111-1111-1111-111111111111  -accel qtest
 # starting QEMU: exec ./qemu-system-x86_64 -qtest unix:/tmp/qtest-417639.sock -qtest-log /dev/null -chardev socket,path=/tmp/qtest-417639.qmp,id=char0 -mon chardev=char0,mode=control -display none -audio none -accel kvm -accel tcg -machine pc-q35-9.1, -name target,debug-threads=on -m 150M -serial file:/tmp/migration-test-XPLUN2/dest_serial -incoming tcp:127.0.0.1:0 -drive if=none,id=d0,file=/tmp/migration-test-XPLUN2/bootsect,format=raw -device ide-hd,drive=d0,secs=1,cyls=1,heads=1     -accel qtest
 **
 ERROR:../../devel/qemu/tests/qtest/migration-test.c:1603:test_analyze_script: code should not be reached
 migration-test: ../../devel/qemu/tests/qtest/libqtest.c:240: qtest_wait_qemu: Assertion `pid == s->qemu_pid' failed.
 migration-test: ../../devel/qemu/tests/qtest/libqtest.c:240: qtest_wait_qemu: Assertion `pid == s->qemu_pid' failed.
 ok 2 /x86_64/migration/analyze-script
 ...

Let's better fail the test in case the child did not exit properly, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>