Consolidate the logic for checking whether a vcpu index is valid.  Also, use
likely(), as a valid value should be the overwhelmingly common case.
Signed-off-by: NJames Morris <jmorris@namei.org>
Signed-off-by: NAvi Kivity <avi@qumranet.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>