KVM: SVM: Move SEV VMCB tracking allocation to sev.c

Move the allocation of the SEV VMCB array to sev.c to help pave the way
toward encapsulating SEV enabling wholly within sev.c.

No functional change intended.

Reviewed by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210422021125.3417167-13-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>