Fix gcc warnings

• Remove -Wunused-but-set-variable (newcpu)
• Always return VMEXIT_CONTINUE as the code always set retval to that value.

Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D7119
MFC after: 1 week
Reported by: Jenkins
Reviewed by: grehan (maintainer)
Sponsored by: EMC / Isilon Storage Division