Allow Plymouth to use graphical backends on kernels without VT

This depends on !177 (merged) for input to work After some testing, this seems to be the minimal and simplest changes to get Plymouth to start and show the splash on kernels without VTs

There is some fuzz that might cause the diff to fail in the ply-device-manager.c , so I may need to change it after.

It seems like the source formatting checker is reporting some lines that are bad in ply-utils.c, however it seems that these lines are not the ones I changed