src/collect.ml: Don't die if a domain goes away during lookup

src/collect.ml: Don't die if a domain goes away during lookup

libvirt: VIR_ERR_NO_DOMAIN: VIR_FROM_QEMU: Domain not found: no domain with matching uuid '231eb609-fe6c-4de5-abbc-e1f834457c37'
Raised by primitive operation at Libvirt in file "libvirt.ml" (inlined), line 573, characters 0-1023
Called from Collect.collect.(fun) in file "collect.ml", line 168, characters 18-44
Called from Stdlib__List.map in file "list.ml", line 92, characters 20-23
Called from Stdlib__List.map in file "list.ml", line 92, characters 32-39
Called from Collect.collect in file "collect.ml", line 138, characters 4-1023
Called from Top.main_loop in file "top.ml", line 318, characters 16-29
Called from Main in file "main.ml", line 50, characters 4-19