cpu plugin: Fix parsing of SoftIRQ under Linux.
swap plugin: Make SwapCached optional under Linux.

OpenVZ doesn't provide the line, which caused swap collection to be
skipped entirely on these systems. Instead, "cached" should simply be
ignored, as it is on other systems. Thanks to @zerkms for reporting this
problem!

Fixes: #733
virt plugin: Rename the "libvirt" plugin to "virt".

"libvirt" confused libtool and caused problems when re-building from source.