KVM: SEV: Move SEV-specific VM initialization to sev.c

KVM: SEV: Move SEV-specific VM initialization to sev.c

Move SEV+ VM initialization to sev.c (as sev_vm_init()) so that
kvm_sev_info (and all usage) can be gated on CONFIG_KVM_AMD_SEV=y without
needing more #ifdefs.  As a bonus, isolating the logic will make it easier
to harden the flow, e.g. to WARN if the vm_type is unknown.

No functional change intended (SEV, SEV_ES, and SNP VM types are only
supported if CONFIG_KVM_AMD_SEV=y).

Link: https://patch.msgid.link/20260310234829.2608037-12-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>