x86: Demacro CONFIG_PARAVIRT cpu accessors

Recently, we had a build failure on !CONFIG_PARAVIRT due to a
callback ->wbinvd() clashing with a macro wbinvd().

While we worked around the issue, avoid it in the future by
changing the macro (and a few surrounding ones) to an inline
function.

Signed-off-by: Avi Kivity <avi@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: http://lkml.kernel.org/r/1303632711-21662-1-git-send-email-avi@redhat.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>