qemu: Prevent crash of libvirtd without guest agent configuration

qemu: Prevent crash of libvirtd without guest agent configuration

If users haven't configured guest agent then qemuAgentCommand() will
dereference a NULL 'mon' pointer, which causes crash of libvirtd when
using agent based cpu (un)plug.

With the patch, when the qemu-ga service isn't running in the guest,
a expected error "error: Guest agent is not responding: Guest agent
not available for now" will be raised, and the error "error: argument
unsupported: QEMU guest agent is not configured" is raised when the
guest hasn't configured guest agent.

GDB backtrace:

 (gdb) bt
 #0  virNetServerFatalSignal (sig=11, siginfo=<value optimized out>, context=<value optimized out>) at rpc/virnetserver.c:326
 #1  <signal handler called>
 #2  qemuAgentCommand (mon=0x0, cmd=0x7f39300017b0, reply=0x7f394b090910, seconds=-2) at qemu/qemu_agent.c:975
 #3  0x00007f39429507f6 in qemuAgentGetVCPUs (mon=0x0, info=0x7f394b0909b8) at qemu/qemu_agent.c:1475
 #4  0x00007f39429d9857 in qemuDomainGetVcpusFlags (dom=<value optimized out>, flags=9) at qemu/qemu_driver.c:4849
 #5  0x00007f3957dffd8d in virDomainGetVcpusFlags (domain=0x7f39300009c0, flags=8) at libvirt.c:9843

How to reproduce?

 # To start a guest without guest agent configuration
 # then run the following cmdline

 # virsh vcpucount foobar --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

RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=984821

Signed-off-by: Alex Jia <ajia@redhat.com>
Signed-off-by: Peter Krempa <pkrempa@redhat.com>