qtest: fix infinite loop when QEMU aborts abruptly

From Markus:

Makes "make check" hang:

    QTEST_QEMU_BINARY=x86_64-softmmu/qemu-system-x86_64 gtester -k --verbose -m=quick tests/crash-test tests/rtc-test
    TEST: tests/crash-test... (pid=972)
    qemu-system-x86_64: Device needs media, but drive is empty
[Nothing happens, wait a while, then hit ^C]
    make: *** [check-qtest-x86_64] Interrupt

This was due to the fact that we weren't checked for errors when
reading from the QMP socket.  This patch adds appropriate error
checking.
Reported-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

tests/libqtest.c

@@ -291,6 +291,11 @@ void qtest_qmp(QTestState *s, const char *fmt, ...)
             continue;
         }
 
+        if (len == -1 || len == 0) {
+            fprintf(stderr, "Broken pipe\n");
+            exit(1);
+        }
+
         switch (c) {
         case '{':
             nesting++;