docs: Document that 'vcpu.delay' statistic is in nanoseconds

Add nanoseconds units for vcpu.delay doc, as it's based on
'/proc/<pid>/task/<tid>/schedstat' (see 'qemuGetSchedstatDelay()').

'schedstat' is in nanoseconds, according to
https://docs.kernel.org/scheduler/sched-stats.html#proc-pid-schedstat.

Signed-off-by: aadmi <aadmi@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>

docs/manpages/virsh.rst

@@ -2441,7 +2441,7 @@ When selecting the *--state* group the following fields are returned:
   no (may indicate the processor is idle or even disabled,
   depending on the architecture)
 * ``vcpu.<num>.delay`` - time the vCPU <num> thread was enqueued by the
-  host scheduler, but was waiting in the queue instead of running.
+  host scheduler, but was waiting in the queue instead of running (in nanoseconds).
   Exposed to the VM as a steal time.
 
 This group of statistics also reports additional hypervisor-originating per-vCPU

src/libvirt-domain.c

@@ -12331,7 +12331,7 @@ virConnectGetDomainCapabilities(virConnectPtr conn,
  *     "vcpu.<num>.delay" - time the vCPU <num> thread was enqueued by the
  *                          host scheduler, but was waiting in the queue
  *                          instead of running. Exposed to the VM as a steal
- *                          time.
+ *                          time. (in nanoseconds)
  *
  *    This group of statistics also reports additional hypervisor-originating
  *    per-vCPU stats. The hypervisor-specific statistics in this group have the