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

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: ef2f11454c(hw/loongarch/virt: Replace Loongson IPI with LoongArch IPI)
Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>