qubesvm: make initial qmemman request consistent with libvirt config

If HVM have PCI device, it can't use PoD, so need 'maxmem' memory to be
started. Request that much from qmemman.
Note that is is somehow independent of enabling or not dynamic memory
management for the VM (`service.meminfo-writer` feature). Even if VM
initially had assigned maxmem memory, it can be later ballooned down.

QubesOS/qubes-issues#3207

qubes/vm/qubesvm.py

@@ -1244,7 +1244,12 @@ class QubesVM(qubes.vm.mix.net.NetVMMixin, qubes.vm.BaseVM):
             else:
                 stubdom_mem = 0
 
-            mem_required = int(self.memory + stubdom_mem) * 1024 * 1024
+            initial_memory = self.memory
+            if self.virt_mode == 'hvm' and self.devices['pci'].persistent():
+                # HVM with PCI devices does not support populate-on-demand on
+                #  Xen
+                initial_memory = self.maxmem
+            mem_required = int(initial_memory + stubdom_mem) * 1024 * 1024
 
         qmemman_client = qubes.qmemman.client.QMemmanClient()
         try: