details: Don't replay boot buffer on serial consoles

details: Don't replay boot buffer on serial consoles

commit 0e59dde8 changed the details plugin to clear the terminal when
first opening it. This was done to prevent duplicate messages from
showing up when toggling back and forth between details and graphical
splashes.

That has the negative side effect of purging serial console output too
though. Furthermore, it makes little sense to replay the boot buffer
on serial consoles, since serial consoles don't aggressively purge
scrollback like VTs do.

This commit adds a check to make sure the terminal is a VT before trying
to clear and replay the scrollback buffer.

Closes: https://gitlab.freedesktop.org/plymouth/plymouth/-/issues/187