qdev: Introduce PropertyInfo.create

This allows property implementation to provide a specialized property
creation method.

Update conditions guarding property types accordingly.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <20170714021509.23681-3-famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
