qemu-file: Add copyright header to qemu-file.c

qemu-file: Add copyright header to qemu-file.c

The person who created qemu-file.c (me, on commit
093c455a8c6d8f715eabd8c8d346f08f17d686ec) didn't add a copyright/license
header to the file, even though the whole code was copied from savevm.c
(which had a copyright/license header).

To correct this, copy the copyright information and license from
savevm.c, that's where the original code came from.

Luckily, very few changes were made on qemu-file.c after it was created.
All the authors who touched the code are being CCed, so they can confirm
if they are OK with the copyright/license information being added.

Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Alexey Kardashevskiy <aik@ozlabs.ru>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Juan Quintela <quintela@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>