migrate.c: migrate_fd_put_buffer: Do not busyloop: stop writing if EWOULDBLOCK

The migration code is non-blocking, designed for live migration.

Practically migrate_fd_put_buffer busy-loops trying to write, as
on many machines EWOULDBLOCK==EAGAIN (look in include/asm-generic/errno.h).

Signed-off-by: Uri Lublin <uril@redhat.com>
