/dev/[kvm|vhost-vsock] are owned by system

/dev/kvm and /dev/vhost-vsock are used by crosvm. Previously, it ran as
a custom UID `virtualizationservice`. However, this prevented us from
applying task profiles to the crosvm process because joining a process
to a cgroup requires system UID.

Now, crosvm (and its parent virtualizationservice as well) runs in
system UID. Therefore, the ownership of two device files are also
updated accorgly.

BUG=b:216788146
BUG=b:223790172
Test: watch TH

Change-Id: I1f63a12532d3a2cb5724291dbbb40210bd7c9203