PyPlugins not always unloaded

[AndrewFasano]

After #1440 we need to ensure pyplugins are unloaded on shutdown - adding this to any loaded pypanda script seems to make it work, but I haven't figure out how to get it into the core of pypanda.

    @panda.cb_pre_shutdown
    def pre_shutdown():
        '''
        Ensure pyplugins nicely clean up. Working around some panda bug
        '''
        panda.pyplugins.unload_all()

[Kingsdom005]

When I run the command "/home/user0/Desktop/PANDA/panda3/panda/build/i386-softmmu/panda-system-i386 -m 8192 -replay test -panda osi -os windows-32-7sp1 -panda win7x86intro -panda osi_test"，things go wrong with message "PANDA[win7x86intro]:Failed to resolve path for plugin."，so how to change the command to deal with this problem?