hw/i386: Replace magic number with field length calculation

Replce the literal magic number 48 with length calculation (32 bytes at
the end of the firmware after the table footer + 16 bytes of the OVMF
table footer GUID).

No functional change intended.

Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220222071906.2632426-3-dovmurik@linux.ibm.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

hw/i386/pc_sysfw_ovmf.c

@@ -30,6 +30,8 @@
 
 #define OVMF_TABLE_FOOTER_GUID "96b582de-1fb2-45f7-baea-a366c55a082d"
 
+static const int bytes_after_table_footer = 32;
+
 static bool ovmf_flash_parsed;
 static uint8_t *ovmf_table;
 static int ovmf_table_len;
@@ -53,12 +55,13 @@ void pc_system_parse_ovmf_flash(uint8_t *flash_ptr, size_t flash_size)
 
     /*
      * if this is OVMF there will be a table footer
-     * guid 48 bytes before the end of the flash file.  If it's
-     * not found, silently abort the flash parsing.
+     * guid 48 bytes before the end of the flash file
+     * (= 32 bytes after the table + 16 bytes the GUID itself).
+     * If it's not found, silently abort the flash parsing.
      */
     qemu_uuid_parse(OVMF_TABLE_FOOTER_GUID, &guid);
     guid = qemu_uuid_bswap(guid); /* guids are LE */
-    ptr = flash_ptr + flash_size - 48;
+    ptr = flash_ptr + flash_size - (bytes_after_table_footer + sizeof(guid));
     if (!qemu_uuid_is_equal((QemuUUID *)ptr, &guid)) {
         return;
     }