Libgcrypt 1.8.0 added support for the XTS mode. Use this because long
term we wish to delete QEMU's XTS impl to avoid carrying private crypto
algorithm impls.

As an added benefit, using this improves performance from 531 MB/sec to
670 MB/sec, since we are avoiding several layers of function call
indirection.

This is even more noticable with the gcrypt builds in Fedora or RHEL-8
which have a non-upstream patch for FIPS mode which does mutex locking.
This is catastrophic for encryption performance with small block sizes,
meaning this patch improves encryption from 240 MB/sec to 670 MB/sec.
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: NStefano Garzarella <sgarzare@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>