Separate the QEMUFile interface from the implementation, to reduce
dependencies from code using QEMUFile.

All the code that is being moved to the new file is exactly the same
code that was on savevm.c (moved by commit
093c455a), so I am using the copyright
and license header from savevm.c for the new file.
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NJuan Quintela <quintela@redhat.com>