data handler: add open method

Add open method to data handler object.  The exact purpose of this
method will vary by transport mechanism.

Change-Id: I3d1fa3914e010ac7590455ae3f948a1d9d5cee4b
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/pci_handler.hpp b/pci_handler.hpp
index 2594f09..ba09d51 100644
--- a/pci_handler.hpp
+++ b/pci_handler.hpp
@@ -22,6 +22,7 @@
   public:
     PciDataHandler() = default;
 
+    bool open() override;
     std::vector<std::uint8_t> copyFrom(std::uint32_t length) override;
     bool write(const std::vector<std::uint8_t>& configuration) override;
     std::vector<std::uint8_t> read() override;