vport: Move 'extern' declarations of vports to header.

vport: Move 'extern' declarations of vports to header.

Since vport implementations have no header files they needed to be
declared as extern before being used.  They are currently declared
in vport.c but this isn't safe because the compiler will silently
accept it if the type is incorrect.  This moves those declarations
into vport.h, which is included by all implementations and will
cause errors about conflicting types if there is a mismatch.