With the latest 3.99.x (updated two minutes ago) I get a different panic
when starting the 7th VM:

cpu_Debugger(...)
panic(c059bea0,ffffffff,0,801727,cae8334c) at panic+0x121
find_unbound_irq(c061ab40,c0d05f00,caeb3ba4,c061ab44,c061ab40) at 
find_unbound_irq+0x2b
bind_evtchn_to_irq(24,7,0,4,4000) at bind_evtchn_to_irq+0xad
xbdback_ctrlif_rx(...)
__ctrl_if_rxmsg_deferred(...)
softintr_dispatch(...)