x86: centralize setting SWIOTLB_FORCE when guest memory encryption is enabled

Move enabling SWIOTLB_FORCE for guest memory encryption into common code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Tested-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>