migration: Use strnlen() for fixed-size string

migration: Use strnlen() for fixed-size string

GCC 8 introduced the -Wstringop-overflow, which detect buffer overflow
by string-modifying functions declared in <string.h>, such strncpy(),
used in global_state_store_running().

GCC indeed found an incorrect use of strlen(), because this array
is loaded by VMSTATE_BUFFER(runstate, GlobalState) then parsed
using qapi_enum_parse which does not get the buffer length.

Use strnlen() which returns sizeof(s->runstate) if the array is not
NUL-terminated, assert the size is within range, and enforce the array
to be NUL-terminated to avoid an overflow in qapi_enum_parse().

This fixes:

    CC      migration/global_state.o
  qemu/migration/global_state.c: In function 'global_state_pre_save':
  qemu/migration/global_state.c:109:15: error: 'strlen' argument 1 declared attribute 'nonstring' [-Werror=stringop-overflow=]
       s->size = strlen((char *)s->runstate) + 1;
                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~
  qemu/migration/global_state.c:24:13: note: argument 'runstate' declared here
       uint8_t runstate[100] QEMU_NONSTRING;
               ^~~~~~~~
  cc1: all warnings being treated as errors
  make: *** [qemu/rules.mak:69: migration/global_state.o] Error 1

Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>