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: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
