KVM: selftests: Add GUEST_SYNC[1-6] macros for synchronizing more data

Add GUEST_SYNC[1-6]() so that tests can pass the maximum amount of
information supported via ucall(), without needing to resort to shared
memory.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20231027182217.3615211-31-seanjc@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>