This significantly speeds up the test suite.
They can't really be easily represented anyways in the test suite
since the devnode would have to change so much between steps.
Although the same payload is used for all 3 MST hubs in TB16 or WD15
it's possible this may change one day. Create separate GUID's for
all 3 different combinations by encoding the chip ID into the GUID
string.
Although WD15 and TB16 share a same payload today, this may not
be the case some day in the future. Use the fu-dell-common method
to determine the dock in use to build the GUID.
There was a lot of confusion about when remote control was turned
on or off leading to dsyfunctional cascade support.
This has been confirmed to work for querying and flashing both
VMM3320s in a Rugged Latitude 5414 + dock.
The synapticsmst-common.c file had some global state so that cascade devices
could use the device fd. This made the control flow error prone, and it meant
that the fd could be leaked trivially on any error path.
Moving the fd ownership to the device is the logical place for the control, and
means we can create "connections" to access the main device and the cascade
devices.
This fixes the warnings detected by Coverity.
