pci: Let pci_dma_read() propagate MemTxResult

pci_dma_rw() returns a MemTxResult type.
Do not discard it, return it to the caller.

Reviewed-by: Li Qiang <liq3ea@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20201023151923.3243652-10-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>