KVM: SEV: Do not allow intra-host migration/mirroring of SNP VMs

KVM: SEV: Do not allow intra-host migration/mirroring of SNP VMs

The intra-host migration/mirroring feature is not fully implemented for
SEV-SNP VMs. The proper migration requires additional SNP-specific
state such as guest_req_mutex, guest_req_buf, and guest_resp_buf to be
transferred or initialized on the destination.

The SNP VM mirroring requires vmsa features to be copied as well otherwise
ASID would be bound to SNP range while VM is detected as a SEV VM.

Reject SNP source VMs in migration/mirroring until proper SNP state
transfer is implemented.

Fixes: 1dfe571c12cf ("KVM: SEV: Add initial SEV-SNP support")
Reported-by: Chris Mason <clm@meta.com>
Reported-by: Sashiko <sashiko-bot@kernel.org>
Assisted-by: Claude:claude-opus-4-6
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Atish Patra <atishp@meta.com>
Link: https://patch.msgid.link/20260602-sev_snp_fixes-v3-1-24bfd3ae047c@meta.com
Cc: stable@vger.kernel.org
[sean: let lines poke past 80 chars, tag for stable]
Signed-off-by: Sean Christopherson <seanjc@google.com>