vl: Rewrite a fall through comment

vl: Rewrite a fall through comment

GCC9 is confused by this comment when building with CFLAG
-Wimplicit-fallthrough=2:

  vl.c: In function ‘qemu_ref_timedate’:
  vl.c:773:15: error: this statement may fall through [-Werror=implicit-fallthrough=]
    773 |         value -= rtc_realtime_clock_offset;
        |         ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
  vl.c:775:5: note: here
    775 |     case QEMU_CLOCK_VIRTUAL:
        |     ^~~~
  cc1: all warnings being treated as errors

Rewrite the comment using 'fall through' which is recognized by
GCC and static analyzers.

Reported-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-Id: <20190719131425.10835-7-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>