dispvm: set 'memory' to the size reported on savefile generation
dispvm: set 'memory' to the size reported on savefile generation

That parameter will be used later to request memory from qmemman just
before loading savefile to memory, so it should match the real need.

Do not allow values smaller than 400, to prevent storing some erroneous
values.

Fixes qubesos/qubes-issues#973

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