accel: Report unknown accelerator as "not found" instead of "does not exist"

accel: Report unknown accelerator as "not found" instead of "does not exist"

As the accelerator classes won't be registered anymore if they are not
enabled at compile time, saying "does not exist" may be misleading, as
the accelerator may be simply disabled. Change the wording to just say
"not found".

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>