There is no need to save and restore the host's return address in the
'struct vmxctx'. It is preserved on the host stack across a guest entry
and exit and just restoring the host's '%rsp' is sufficient.

Pointed out by:	grehan@
Fix insta-panic on assert of unlocked periph mtx in ciss(4) when
logical volume state changes.

Currently, I view this as a critical fix for users and will MFC this rapidly as
my testing has shown data loss when the disk is failed by removing it when
under some amount of write activity and this code panics the box.

Reviewed by:	mav@ scottl@
MFC after:	3 days
Sponsored by:	Yahoo! Inc.