vppapigen: do not allow to define message as both request and reply
vppapigen: do not allow to define message as both request and reply

Change-Id: I0738fac32dd0c5d927c52d2eb2c1100d14c147cf
Signed-off-by: Marek Gradzki <3ca09cd7b7147613247c2a96dcdb0ba33114f8e7@cisco.com>

chrisy/vpp,vpp-dev/vpp,vpp-dev/vpp,vpp-dev/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,vpp-dev/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,vpp-dev/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,vpp-dev/vpp,vpp-dev/vpp,FDio/vpp,chrisy/vpp