Let devices specify transaction attributes when calling ld*_pci_dma().

Keep the default MEMTXATTRS_UNSPECIFIED in the few callers.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20211223115554.3155328-22-philmd@redhat.com>