migration/postcopy: Cache the tid->vcpu mapping for blocktime

migration/postcopy: Cache the tid->vcpu mapping for blocktime

Looking up the vCPU index for each fault can be expensive when there're
hundreds of vCPUs.  Provide a cache for tid->vcpu instead with a hash
table, then lookup from there.

When at it, add another counter to record how many non-vCPU faults it gets.
For example, the main thread can also access a guest page that was missing.
These kind of faults are not accounted by blocktime so far.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20250613141217.474825-11-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>