loader: add rom transaction API

loader: add rom transaction API

Image file loaders may add a series of roms.  If an error occurs partway
through loading there is no easy way to drop previously added roms.

This patch adds a transaction mechanism that works like this:

  rom_transaction_begin();
  ...call rom_add_*()...
  rom_transaction_end(ok);

If ok is false then roms added in this transaction are dropped.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20180814162739.11814-5-stefanha@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>