Only build libfwupdplugin source files once; we were only using one
non-exported symbol and so move that to the exported map so we can just link
the library in the self tests.
There is no Manager object, so block devices must be discovered
manually.
Additionally, Type field contains FS name rather than its MBR ID in
hex form or a GPT GUID.
Signed-off-by: Richard Hughes <richard@hughsie.com>
