graph-depends: add support for excluding host packages

Just like the --stop-on and --exclude options allow to stop on or
exclude virtual packages from the list by passing the "virtual" magic
value, this commit extends the graph-depends logic to support a "host"
magic value for --stop-on and --exclude. This will allow to draw the
graph by stopping on host packages, or by excluding host packages.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Thomas: minor code beautification suggested by Yann E. Morin.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>