Add burst-proof resource reservations

[sjmiller609]

Summary

• add disk to aggregate resource admission checks
• reserve resources in memory during create/start/restore to prevent burst oversubscription
• add focused resource-manager coverage for disk and pending reservations

Testing

• go test ./lib/resources

Notes

• full lib/instances tests are blocked in this checkout by missing embedded binaries: guest_agent/guest-agent and vz-shim/vz-shim