reduce default SWIOTLB size when no PCI devs are used

Linux inside HVM will allocate 64MB for bouncing DMA (SWIOTLB) by
default. If no real PCI device is assigned, that's way too much, and
wastes over 15% of VM's initial memory.
With real PCI devices, it's usually too much too, but it's very device
specific, so don't risk breaking it. In other cases, reduce default to
4MB.

Note PVH domain will not allocate SWIOTLB anyway, as no PCI devices are
there at all. This difference contributes to the VM start time, so
reducing SWIOTLB should also improve that part.

QubesOS/qubes-issues#6174

qubes/config.py

@@ -52,7 +52,7 @@
     'libvirt_uri': 'xen:///',
     'memory': 400,
     'hvm_memory': 400,
-    'kernelopts': "",
+    'kernelopts': "swiotlb=2048",
     'kernelopts_pcidevs': "",
     'kernelopts_common': ('root=/dev/mapper/dmroot ro nomodeset console=hvc0 '
              'rd_NO_PLYMOUTH rd.plymouth.enable=0 plymouth.enable=0 '),