qmemman: fix list/filter handling on python3
qmemman: fix list/filter handling on python3

Filter in python3 returns a generator, can be iterated only once.
This is about list of existing domains - store it as a list, otherwise
domains will "disappear" after being discovered.

QubesOS/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin