Add qemu_get_counted_string to read a string prefixed by a count byte

and use it in loadvm_state and ram_load.

Where ever it's used, check the return and error if it failed.

Minor: ram_load was using a 257 byte array for its string, the
       maximum length is 255 bytes + 0 terminator, so fix to 256

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

migration/qemu-file.c

@@ -585,3 +585,20 @@ int qemu_put_qemu_file(QEMUFile *f_des, QEMUFile *f_src)
     }
     return len;
 }
+
+/*
+ * Get a string whose length is determined by a single preceding byte
+ * A preallocated 256 byte buffer must be passed in.
+ * Returns: len on success and a 0 terminated string in the buffer
+ *          else 0
+ *          (Note a 0 length string will return 0 either way)
+ */
+size_t qemu_get_counted_string(QEMUFile *f, char buf[256])
+{
+    size_t len = qemu_get_byte(f);
+    size_t res = qemu_get_buffer(f, (uint8_t *)buf, len);
+
+    buf[res] = 0;
+
+    return res == len ? res : 0;
+}