virt plugin: Rename the "libvirt" plugin to "virt".

"libvirt" confused libtool and caused problems when re-building from source.
memory plugin: Add slab memory stats (linux)

Pull-Request: #560
write_graphite plugin: Limit number of reconnection attempts per time.

Fixes: #625
Merge branch 'master' into debian
updating build.sh results
Merge branch "debian" into debian-wheezy-backports