KVM: SEV: Select KVM_GENERIC_PRIVATE_MEM when CONFIG_KVM_AMD_SEV=y

SEV-SNP relies on private memory support to run guests, so make sure to
enable that support via the CONFIG_KVM_GENERIC_PRIVATE_MEM config
option.

Signed-off-by: Michael Roth <michael.roth@amd.com>
Message-ID: <20240501085210.2213060-4-michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>