Add 'r' to register name r2 in kvmppc_hv_enter.

Also update comment at the top of kvmppc_hv_enter to indicate that R2/TOC is
non-volatile.
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>