Doesn’t a malicious actor need to break out of the (App)VM, in order to exploit Meltdown/Spectre, as CPU is virtualized/abstracted?
No.
“Xen guests may be able to infer the contents of arbitrary host memory,
including memory assigned to other guests.” (XSA-254 - Xen Security Advisories)
domU shouldn’t have access to dom0 memory management at all.
Then what will manage dom0’s memory?
Not sure about this one with regards to Qubes OS. Can anybody comment?
As @unman explained earlier, revealing Qubes OS usage is not about TCP/IP level but about the hosts the OS contacts, e.g. during updates.