tests/qtest/libqos: add DMA support for writing and reading fw_cfg files

tests/qtest/libqos: add DMA support for writing and reading fw_cfg files

At present, the libqos/fw_cfg.c library does not support the modern DMA
interface which is required to write to the fw_cfg files. It only uses the IO
interface. Implement read and write methods based on DMA. This will enable
developers to add tests that writes to the fw_cfg file(s). The structure of
the code is taken from edk2 fw_cfg implementation. It has been tested by
writing a qtest that writes to a fw_cfg file.

Signed-off-by: Ani Sinha <anisinha@redhat.com>
Message-ID: <20250120043847.954881-3-anisinha@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>