Avoid a warning from OpenBSD linker

Avoid the warning below by using snprintf:
../libhw64/vl.o(.text+0x78d4): In function `get_boot_devices_list':
/src/qemu/vl.c:763: warning: sprintf() is often misused, please use snprintf()
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

vl.c

@@ -759,8 +759,10 @@ char *get_boot_devices_list(uint32_t *size)
         }
 
         if (i->suffix && devpath) {
-            bootpath = qemu_malloc(strlen(devpath) + strlen(i->suffix) + 1);
-            sprintf(bootpath, "%s%s", devpath, i->suffix);
+            size_t bootpathlen = strlen(devpath) + strlen(i->suffix) + 1;
+
+            bootpath = qemu_malloc(bootpathlen);
+            snprintf(bootpath, bootpathlen, "%s%s", devpath, i->suffix);
             qemu_free(devpath);
         } else if (devpath) {
             bootpath = devpath;