KVM: SEV: Assert that kvm->lock is held when querying SEV+ support

Assert that kvm->lock is held when checking if a VM is an SEV+ VM, as KVM
sets *and* resets the relevant flags when initialization SEV state, i.e.
it's extremely easy to end up with TOCTOU bugs if kvm->lock isn't held.

Add waivers for a VM being torn down (refcount is '0') and for there being
a loaded vCPU, with comments for both explaining why they're safe.

Note, the "vCPU loaded" waiver is necessary to avoid splats on the SNP
checks in sev_gmem_prepare() and sev_gmem_max_mapping_level(), which are
currently called when handling nested page faults.  Alternatively, those
checks could key off KVM_X86_SNP_VM, as kvm_arch.vm_type is stable early
in VM creation.  Prioritize consistency, at least for now, and to leave a
"reminder" that the max mapping level code in particular likely needs
special attention if/when KVM supports dirty logging for SNP guests.

Link: https://patch.msgid.link/20260310234829.2608037-16-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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