xen/privcmd: remove unused variable pageidx

Variable pageidx is assigned a value but it is never read, hence it
is redundant and can be removed. Cleans up clang warning:

drivers/xen/privcmd.c:199:2: warning: Value stored to 'pageidx'
is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>