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

dispvm/qubes-prepare-saved-domain.sh

@@ -58,6 +58,15 @@ if ! virsh -c xen:/// save $1 $2; then
 	exit 1
 fi
 rm -f $QMEMMAN_STOP
+# Do not allow smaller allocation than 400MB. If that small number comes from
+# an error, it would prevent further savefile regeneration (because VM would
+# not start with too little memory). Also 'maxmem' depends on 'memory', so
+# 400MB is sane compromise.
+if [ "$MEM" -lt 409600 ]; then
+    qvm-prefs -s $1 memory 400
+else
+    qvm-prefs -s $1 memory $[ $MEM / 1024 ]
+fi
 ln -snf $VMDIR /var/lib/qubes/dvmdata/vmdir
 cd $VMDIR
 bsdtar -cSf saved-cows.tar volatile.img