spice-vm-install.cfg: Extend inactivity treshold

There is an issue with framebuffer during installation, that
causes the display to go all black, unless there is some user
input. Installation continues just fine

qemu/cfg/spice-vm-install.cfg

@@ -17,6 +17,7 @@ variants:
                 install_timeout = 3600
                 kernel_params = "inst.ks=cdrom nicdelay=60 console=ttyS0,115200 console=tty0 inst.repo=${url}"
                 unattended_file = unattended/RHEL-7-spice.ks
+                inactivity_treshold = 3600
                 only Linux.RHEL.7.devel.x86_64
                 only unattended_install..extra_cdrom_ks.default_install.aio_native
             - Windows-7-x64: