Disallow negative silo quota CPU value

[david-crespo]

Nexus does not prevent you from setting silo quotas to a negative number.

This results in an unfortunate utilization situation:

[hawkw]

Yeah, it probably shouldn't let you do that...

[david-crespo]

Let's try and sneak this into v10.

[david-crespo]

Thanks @augustuswm for pointing out that memory and storage already disallow negative values because they are ByteCounts.

omicron/nexus/types/src/external_api/params.rs

Lines 375 to 385 in 54b3878

	/// Updateable properties of a Silo's resource limits.
	/// If a value is omitted it will not be updated.
	#[derive(Clone, Debug, Deserialize, Serialize, JsonSchema)]
	pub struct SiloQuotasUpdate {
	/// The amount of virtual CPUs available for running instances in the Silo
	pub cpus: Option<i64>,
	/// The amount of RAM (in bytes) available for running instances in the Silo
	pub memory: Option<ByteCount>,
	/// The amount of storage (in bytes) available for disks or snapshots
	pub storage: Option<ByteCount>,
	}