vmm: don't allow resizing to 0 vCPUs

No sane guest OS will allow hotplugging all cpus.
However, the REST API currently allows specifying
`ch-remote resize --cpus 0`.

On Linux, we can then observe the following error in the kernel log:

processor cpu0: Offline failed.

Subsequent resize commands via ch-remote will then fail with
VcpuPendingRemovedVcpu because the removal of cpu0 was never
successful.

Fix this by disallowing resizing to zero vCPUs.

Signed-off-by: Thomas Prescher <thomas.prescher@cyberus-technology.de>
On-behalf-of: SAP thomas.prescher@sap.com

vmm/src/cpu.rs

@@ -157,6 +157,9 @@ pub enum Error {
     #[error("Error adding CpuManager to MMIO bus: {0}")]
     BusError(#[source] vm_device::BusError),
 
+    #[error("Requested zero vCPUs")]
+    DesiredVCpuCountIsZero,
+
     #[error("Requested vCPUs exceed maximum")]
     DesiredVCpuCountExceedsMax,
 
@@ -1320,6 +1323,10 @@ impl CpuManager {
             return Ok(false);
         }
 
+        if desired_vcpus < 1 {
+            return Err(Error::DesiredVCpuCountIsZero);
+        }
+
         if self.check_pending_removed_vcpu() {
             return Err(Error::VcpuPendingRemovedVcpu);
         }