hw/i386/vmport: fix missing definitions with non-log trace backends

When compiled with anything other than the 'log' trace backend, we have:

   error: implicit declaration of function 'qemu_log_mask'
   error: 'LOG_UNIMP' undeclared (first use in this function)

This patch adds the missing include.

Fixes: 7299e1a4
       ("hw/i386/vmport: replace fprintf() by trace events or LOG_UNIMP")
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Message-id: 20171221211103.30311-1-laurent@vivier.eu
[PMM: fixed commit message description of when problem occurs]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>