xen/gntdev: add ioctl for grant copy

Add IOCTL_GNTDEV_GRANT_COPY to allow applications to copy between user
space buffers and grant references.

This interface is similar to the GNTTABOP_copy hypercall ABI except
the local buffers are provided using a virtual address (instead of a
GFN and offset).  To avoid userspace from having to page align its
buffers the driver will use two or more ops if required.

If the ioctl returns 0, the application must check the status of each
segment with the segments status field.  If the ioctl returns a -ve
error code (EINVAL or EFAULT), the status of individual ops is
undefined.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>