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: NBrijesh Singh <brijesh.singh@amd.com>
Signed-off-by: NSean Christopherson <seanjc@google.com>
Message-Id: <20210422021125.3417167-13-seanjc@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>