qemu.cfg.host-kernel: Add 6.8 host kernel.

Signed-off-by: NCong Li <coli@redhat.com>

qemu/cfg/host-kernel/Host_RHEL/6/7.cfg

-- 7:
+- u7:
     host_kernel_ver_str += ".7"
     requires_kernel = [">= 2.6.32-505", "<  2.6.32-574"]
     machine_type = "rhel6.6.0"

qemu/cfg/host-kernel/Host_RHEL/6/8.cfg

+- u8:
+    host_kernel_ver_str += ".8"
+    requires_kernel = [">= 2.6.32-574", "<  2.6.32-643"]
+    machine_type = "rhel6.6.0"
+    set_link:
+        operstate_always_up = no
+    cpuinfo_query.qmachine_type:
+        cpu_info = "pc,rhel.6.6.0,rhel6.5.0,rhel6.4.0,rhel6.3.0,rhel6.2.0,rhel6.1.0,rhel6.0.0,rhel5.5.0,rhel5.4.4,rhel5.4.0"
+    Windows:
+        #hv_relaxed option is supported since RHEL.6.4
+        cpu_model_flags += ",hv_relaxed"