spire: turn on snapshots for autoinstall USB disks

The newer version of qemu provided by default on buster turns on file
locking by default, which prevents us from installing multiple nodes at
the same time. Turn on snapshot mode, so that none of the VMs will
write back changes to the USB drive -- which is what we want, anyway.
This lets us keep installing multiple VMs at the same time.

We unfortunately can't just set the disk to read-only, because IDE
apparently doesn't support that except for CDROMs, so qemu won't let us
enable the option.

platform/spire/src/virt.py

@@ -323,8 +323,9 @@ def generate_qemu_args(self):
             args += ["-drive", "media=disk,file=" + self.hd]
             args += ["-boot", "c"]
         else:
-            # TODO: maybe set this readonly
-            args += ["-drive", "media=%s,format=raw,file=%s" % ("cdrom" if self.cdrom_install else "disk", self.cd)]
+            # snapshot means that the installer drive won't be modifiable by the VMs, which is useful in qemu 3.0.0+ to
+            # avoid locking problems when more than one VM is running at the same time.
+            args += ["-drive", "media=%s,format=raw,file=%s,snapshot=on" % ("cdrom" if self.cdrom_install else "disk", self.cd)]
             args += ["-drive", "media=disk,file=" + self.hd]
             args += ["-boot", "c"]
         args += ["-no-reboot"]