migration: unexport migrate_fd_error()

The only migrate_fd_error() call sites are in "migration/migration.c",
which is also where we define migrate_fd_error(). Make the function
static, and remove its declaration from "migration/migration.h".

Cc: Juan Quintela <quintela@redhat.com> (maintainer:Migration)
Cc: Leonardo Bras <leobras@redhat.com> (reviewer:Migration)
Cc: Peter Xu <peterx@redhat.com> (reviewer:Migration)
Cc: qemu-trivial@nongnu.org
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2018404


Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>