Http Server: Use const for operation names

Operation names are used to mapped operation to path, it is now possible
to use a constant instead of a string. This will result in a compilation
failure instead of a run time failure.

For example instead of using "getFile" to locate the operation, it is
possible to use: file_json::getFile

If for some reason the operation name will changed, the compilation will
fail.

Signed-off-by: Amnon Heiman <amnon@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Http Server: Change file.cc to use constants over string

The code generation now produces constants for the operation name. The
file.cc was modified to use those constants.

Signed-off-by: Amnon Heiman <amnon@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>