Let qemubuilder cache downloaded debs.

[Werner Mahr]

Hi,

here's a little patch to let qemubuilder cache the downloaded debs. By now 
it just copies the debs out of the buildplace before deleting, and copies 
them back when starting a new build.

It works that way for some time now, but it doesn't delete updated packages 
from the cache, but I not sure if I want him to do that.

While implementing this, I fixed the issue the buildplace get a second / in 
some cases.
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