replace vCPU's dirty CPU mask by numeric ID

replace vCPU's dirty CPU mask by numeric ID

At most one bit can be set in the masks, so especially on larger systems
it's quite a bit of unnecessary memory and processing overhead to track
the information as a mask. Store the numeric ID of the respective CPU
instead, or VCPU_CPU_CLEAN if no dirty state exists.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Julien Grall <julien.grall@linaro.org>