Fix initial value of dom0 static-max entry
Fix initial value of dom0 static-max entry

This value needs to be set to actual static max for qmemman to work
properly. If it's set higher than real static-max, qmemman will try to
assign more memory to dom0, which dom0 could not use - will be wasted.
Since this script is executed before any VM is started, simply
take the current dom0 memory usage, instead of parsing dom0_mem Xen
argument. There doesn't seem to be nice API to get this value from Xen
directly.

Fixes QubesOS/qubes-issues#4891

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