x86: rename copy_{from,to}_user() to copy_{from,to}_guest_pv()

x86: rename copy_{from,to}_user() to copy_{from,to}_guest_pv()

Bring them (back) in line with __copy_{from,to}_guest_pv(). Since it
falls in the same group, also convert clear_user(). Instead of adjusting
__raw_clear_guest(), drop it - it's unused and would require a non-
checking __clear_guest_pv() which we don't have.

Add previously missing __user at some call sites and in the function
declarations.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>