dump: add API to write elf notes to buffer

the function can be used by write_elf32_notes/write_elf64_notes to write notes
to a buffer. If fd_write_vmcore is used, write_elf32_notes/write_elf64_notes
will write elf notes to vmcore directly. Instead, if buf_write_note is used,
elf notes will be written to opaque->note_buf at first.

Signed-off-by: Qiao Nuohan <qiaonuohan@cn.fujitsu.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

dump.c

@@ -76,6 +76,9 @@ typedef struct DumpState {
     int64_t begin;
     int64_t length;
     Error **errp;
+
+    uint8_t *note_buf;          /* buffer for notes */
+    size_t note_buf_offset;     /* the writing place in note_buf */
 } DumpState;
 
 static int dump_cleanup(DumpState *s)
@@ -749,6 +752,22 @@ static int write_buffer(int fd, off_t offset, const void *buf, size_t size)
     return 0;
 }
 
+static int buf_write_note(const void *buf, size_t size, void *opaque)
+{
+    DumpState *s = opaque;
+
+    /* note_buf is not enough */
+    if (s->note_buf_offset + size > s->note_size) {
+        return -1;
+    }
+
+    memcpy(s->note_buf + s->note_buf_offset, buf, size);
+
+    s->note_buf_offset += size;
+
+    return 0;
+}
+
 static ram_addr_t get_start_block(DumpState *s)
 {
     GuestPhysBlock *block;