fix(instances): new q35 VMs have spare PCIe ports, and a full VM says what to do

On a q35 machine each PCIe device (a virtio disk, a network card) takes
a root port of its own, libvirt makes only as many ports as the VM is
defined with, and a running VM gets no new one: adding a virtio disk to
such a VM failed with libvirt's "No more available PCI slots".

A new q35 VM is now defined with the PCIe root and 14 root ports, as
virt-install makes them: libvirt puts the VM's devices into them and
leaves the rest free for devices added while it runs.

When a running VM has no free slot, Add disk and Add network device say
what to do instead of libvirt's words: for a disk, the scsi bus when
the VM has a SCSI controller (its disks need no slot of their own), or
adding it with the VM shut down, where libvirt makes the port; for a
network device, shutting the VM down. A new volume whose attach was
refused stays in its pool, as before, to attach as an existing volume.

vrtManager/create.py

@@ -9,6 +9,8 @@ from vrtManager.connection import wvmConnect
 
 # libvirt's CPU modes a new VM can have; any other value is a CPU model's name (mode custom)
 CPU_MODES = ("host-model", "host-passthrough", "maximum")
+# the PCIe root ports of a new q35 VM, its devices' and the spare ones together
+PCIE_ROOT_PORTS = 14
 
 
 def cpu_choices(dom_caps):
@@ -379,7 +381,14 @@ class wvmCreate(wvmConnect):
 
         xml += f""" <video>
                       <model type='{video}'/>
-                   </video>
+                   </video>"""
+        # q35 plugs each PCIe device (a virtio disk, a NIC) into a root port of
+        # its own and libvirt makes only as many as the VM is defined with: as
+        # virt-install, 14 ports, so a running VM can take more devices
+        if "q35" in machine:
+            xml += "<controller type='pci' model='pcie-root'/>"
+            xml += "<controller type='pci' model='pcie-root-port'/>" * PCIE_ROOT_PORTS
+        xml += """
               </devices>
             </domain>"""
         return self._defineXML(xml)