Monitor/QMP: Don't allow cont on bad VM state

We have two states where issuing cont before system_reset can
cause problems: RSTATE_SHUTDOWN (when -no-shutdown is used) and
RSTATE_PANICKED (which only happens with kvm).

This commit fixes that by doing the following when state is
RSTATE_SHUTDOWN or RSTATE_PANICKED:

 1. returning an error to the user/client if cont is issued
 2. automatically transition to RSTATE_PAUSED during system_reset

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

qerror.c

@@ -193,6 +193,10 @@ static const QErrorStringTable qerror_table[] = {
         .error_fmt = QERR_QMP_EXTRA_MEMBER,
         .desc      = "QMP input object member '%(member)' is unexpected",
     },
+    {
+        .error_fmt = QERR_RESET_REQUIRED,
+        .desc      = "Resetting the Virtual Machine is required",
+    },
     {
         .error_fmt = QERR_SET_PASSWD_FAILED,
         .desc      = "Could not set password",