qemu-file: Move stdio implementation to qemu-file-stdio.c

qemu-file: Move stdio implementation to qemu-file-stdio.c

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

The code that is being moved is similar to the one that was on savevm.c before
it was moved in commit 093c455a8c6d8f715eabd8c8d346f08f17d686ec, except for
some changes done by Markus, Juan, and myself. So, I am using the copyright and
license header from savevm.c, but CCing Juan and Markus so they can review the
copyright/license header.

Cc: Markus Armbruster <armbru@redhat.com>
Cc: Juan Quintela <quintela@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>