hw/loongarch: Rename file 'loongson3.XXX' to 'virt.XXX'

1. Rename 'loongson3.c' to 'virt.c' and change the meson.build file.
2. Rename 'loongson3.rst' to 'virt.rst'.

Signed-off-by: Xiaojuan Yang <yangxiaojuan@loongson.cn>
Message-Id: <20220729073018.27037-2-yangxiaojuan@loongson.cn>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

hw/loongarch/meson.build

@@ -2,7 +2,7 @@ loongarch_ss = ss.source_set()
 loongarch_ss.add(files(
     'fw_cfg.c',
 ))
-loongarch_ss.add(when: 'CONFIG_LOONGARCH_VIRT', if_true: [files('loongson3.c'), fdt])
+loongarch_ss.add(when: 'CONFIG_LOONGARCH_VIRT', if_true: [files('virt.c'), fdt])
 loongarch_ss.add(when: 'CONFIG_ACPI', if_true: files('acpi-build.c'))
 
 hw_arch += {'loongarch': loongarch_ss}