qemu: fix uninitialised variable in virQEMUDriverConfigLoadFile

qemu: fix uninitialised variable in virQEMUDriverConfigLoadFile

Since virConfGetValueBool() can return earlier, the parameter 'value'
might be not initialised properly inside this method. Another proof:
Valgrind is returning this error during the libvirtd daemon startup:

==16199== Conditional jump or move depends on uninitialised value(s)
==16199==    at 0x27FFFEF4: virQEMUDriverConfigLoadFile (qemu_conf.c:809)
==16199==    by 0x2807665C: qemuStateInitialize (qemu_driver.c:654)
==16199==    by 0x5535428: virStateInitialize (libvirt.c:662)
==16199==    by 0x12AED8: daemonRunStateInit (remote_daemon.c:802)
==16199==    by 0x536DE18: virThreadHelper (virthread.c:206)
==16199==    by 0x6CB36DA: start_thread (pthread_create.c:463)
==16199==    by 0x6FEC88E: clone (clone.S:95)

Signed-off-by: Julio Faracco <jcfaracco@gmail.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Ján Tomko <jtomko@redhat.com>