save screendump per VM pid rather then name

currently we save VM screen dump into diff dirs by their names,
like VM1 VM2(if that case needs 2 VMs)
but this brings a little bit confusion:

1) migration:
from screendumps, we have nowhere(or very hard to figure out) to know which picture is produced by which stage
VM1(pid 100) -> VM2(pid 110), so they save pics into dir1 dir2, Okay, now VM1 quit, and migration goes vesa direction:
VM2(pid 110, this didn't quit) -> VM1(pid 120), and contiue save pics into dir2 dir1.
but ! now the VM1 is not the one at the beginning !

2) we are running a case w/ VM live from last case:
say the last case using a virtio-net, and this one using a e1000, in the current design both VMs would named VM1, thus two individual VM's screendump are saved together, this is not cool, they have even diff hw !

3) Win install fail and copied img:
as our autotest would copy a img from NFS when installation fails and then boot the img to verify, like the 2), both VMs are named VM1, this brings another confusion when debugging an issue.