KVM: x86: support CONFIG_KVM_AMD=y with CONFIG_CRYPTO_DEV_CCP_DD=m

SEV requires access to the AMD cryptographic device APIs, and this
does not work when KVM is builtin and the crypto driver is a module.
Actually the Kconfig conditions for CONFIG_KVM_AMD_SEV try to disable
SEV in that case, but it does not work because the actual crypto
calls are not culled, only sev_hardware_setup() is.

This patch adds two CONFIG_KVM_AMD_SEV checks that gate all the remaining
SEV code; it fixes this particular configuration, and drops 5 KiB of
code when CONFIG_KVM_AMD_SEV=n.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>