guest-utils.service: add ConditionVirtualization=oracle

Will Thompsonrequested to merge
wjt/virtualbox:ConditionVirtualization-oracle into master

This avoids launching the .init script at all in the case when virtualbox-guest-utils is installed but the system is not a VirtualBox VM. This situation is rare in Debian, but every Endless OS system includes virtualbox-guest-utils, and the majority of computers are not VirtualBox VMs.

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