Description of problem:
If users haven't configured guest agent then qemuAgentCommand() will
dereference a NULL 'mon' pointer, which causes crash of libvirtd.

Version-Release number of selected component (if applicable):
# rpm -q libvirt qemu-kvm kernel
libvirt-0.10.2-20.el6.x86_64
package qemu-kvm is not installed
kernel-2.6.32-288.el6.x86_64

Notes, need to bypass bug 984793 and successfully start a guest firstly.

How reproducible:
always

Steps to Reproduce:
1. virsh start <domain>  (without guest agent configuration)
2. virsh vcpucount <domain> --guest


Actual results:
# virsh vcpucount hello --guest
error: End of file while reading data: Input/output error
error: One or more references were leaked after disconnect from the hypervisor
error: Failed to reconnect to the hypervisor

Expected results:
fix it.

Additional info:

Thread 11 (Thread 0x7f394b091700 (LWP 25677)):
#0  virNetServerFatalSignal (sig=11, siginfo=<value optimized out>, context=<value optimized out>) at rpc/virnetserver.c:326
        sig_action = {__sigaction_handler = {sa_handler = 0x70, sa_sigaction = 0x70}, sa_mask = {__val = {139884217688997, 206158430248, 139884048746048, 206158430256, 139884048746120, 206158430256, 
              139884048746136, 139884048745936, 139884048745520, 1, 139884048745648, 28, 139884265387128, 139884048746208, 139884218222960, 139887012315137}}, sa_flags = 805312336, 
          sa_restorer = 0x7f3930000020}
        origerrno = <value optimized out>
#1  <signal handler called>
No symbol table info available.
#2  qemuAgentCommand (mon=0x0, cmd=0x7f39300017b0, reply=0x7f394b090910, seconds=-2) at qemu/qemu_agent.c:975
        ret = <value optimized out>
        msg = {txBuffer = 0x0, txOffset = 941387424, txLength = 32569, rxBuffer = 0x7f394b090890 "\240\b\tK9\177", rxLength = 1898801172, rxObject = 0x0, finished = 224}
        cmdstr = 0x7f394b0908d0 "\240n\034\070\071\177"
        await_event = <value optimized out>
        __FUNCTION__ = "qemuAgentCommand"
        __func__ = "qemuAgentCommand"
#3  0x00007f39429507f6 in qemuAgentGetVCPUs (mon=0x0, info=0x7f394b0909b8) at qemu/qemu_agent.c:1475
        ret = -1
        i = <value optimized out>
        cmd = 0x7f39300017b0
        reply = 0x0
        data = 0x0
        ndata = <value optimized out>
        __FUNCTION__ = "qemuAgentGetVCPUs"
#4  0x00007f39429d9857 in qemuDomainGetVcpusFlags (dom=<value optimized out>, flags=9) at qemu/qemu_driver.c:4849
        driver = 0x7f3938009840
        priv = 0x7f39381c6ea0
        vm = 0x7f39381c6d40
        def = 0x7f39381d3a00
        ret = -1
        caps = 0x7f39381c4fe0
        cpuinfo = 0x0
        ncpuinfo = -1
        i = <value optimized out>
        __FUNCTION__ = "qemuDomainGetVcpusFlags"
#5  0x00007f3957dffd8d in virDomainGetVcpusFlags (domain=0x7f39300009c0, flags=8) at libvirt.c:9843
        ret = <value optimized out>
        conn = <value optimized out>
        __func__ = "virDomainGetVcpusFlags"
        __FUNCTION__ = "virDomainGetVcpusFlags"
#6  0x00007f395882dceb in remoteDispatchDomainGetVcpusFlags (server=<value optimized out>, client=<value optimized out>, msg=<value optimized out>, rerr=0x7f394b090ba0, args=0x7f3930000a00, 
    ret=0x7f3930001e60) at remote_dispatch.h:4343
        rv = -1
        dom = 0x7f39300009c0
        num = <value optimized out>
        priv = <value optimized out>