Update Xen's memory overhead formula

Experiments show that using memory hotplug or populate-on-demand makes
no difference in required memory at startup. But PV vs PVH/HVM does make
a difference - PV doesn't need extra per-MB overhead at all.

On top of that, experimentally find the correct factor. Do it by
starting VM (paused) with different parameters and compare `xl info
free_memory` before and after.

memory / maxmem: difference
400  / 4000: 434
600  / 4000: 634
400  / 400: 405
400  / 600: 407
400  / 2000: 418
2000 / 2000: 2018
600  / 600: 607

All above are with 2 vcpus. Testing with other vcpus count shows the
1.5MB per vcpu is quite accurate.
As seen above, the initial memory doesn't affect the overhead. The
maxmem counts. Applying linear regression to that shows it's about
0.008MB per MB of maxmem, so round it up to 8192 bytes.
The base overhead of 4MB doesn't match exactly, but since the calculated
number is smaller, leave it at 4MB as a safety margin.

Fixes QubesOS/qubes-issues#9431

qubes/vm/qubesvm.py

@@ -1767,10 +1767,12 @@ def request_memory(self, mem_required=None):
         try:
             mem_required_with_overhead = mem_required + MEM_OVERHEAD_BASE \
                                          + self.vcpus * MEM_OVERHEAD_PER_VCPU
-            if self.use_memory_hotplug:
-                # extra overhead to account future hotplug memory
-                # 1 page per 1MB of RAM, see libxl__get_required_paging_memory()
-                mem_required_with_overhead += self.maxmem * 4096
+            maxmem = self.maxmem if self.maxmem else self.memory
+            if self.virt_mode != "pv":
+                # extra overhead to account (possibly future hotplug) memory
+                # 2 pages per 1MB of RAM, see
+                # libxl__get_required_paging_memory()
+                mem_required_with_overhead += maxmem * 8192
             got_memory = qmemman_client.request_memory(
                 mem_required_with_overhead)