hw/loongarch/virt: Add description for virt machine type

The description about virt machine type is removed by mistake, add
new description here. Here is output result with command
"./qemu-system-loongarch64 -M help"

Supported machines are:
none                 empty machine
virt                 QEMU LoongArch Virtual Machine (default)
x-remote             Experimental remote machine

Without the patch, it shows as follows:
Supported machines are:
none                 empty machine
virt                 (null) (default)
x-remote             Experimental remote machine

Fixes: ef2f114(hw/loongarch/virt: Replace Loongson IPI with LoongArch IPI)
Signed-off-by: Bibo Mao <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Reviewed-by: Michael Tokarev <[email protected]>
Signed-off-by: Michael Tokarev <[email protected]>
(cherry picked from commit 4265b4f)
Signed-off-by: Michael Tokarev <[email protected]>

hw/loongarch/virt.c

@@ -1390,6 +1390,7 @@ static void virt_class_init(ObjectClass *oc, void *data)
     mc->init = virt_init;
     mc->default_cpu_type = LOONGARCH_CPU_TYPE_NAME("la464");
     mc->default_ram_id = "loongarch.ram";
+    mc->desc = "QEMU LoongArch Virtual Machine";
     mc->max_cpus = LOONGARCH_MAX_CPUS;
     mc->is_default = 1;
     mc->default_kernel_irqchip_split = false;