x86: package up context switch hook pointers

x86: package up context switch hook pointers

They're all solely dependent on guest type, so we don't need to repeat
all the same three pointers in every vCPU control structure. Instead use
static const structures, and store pointers to them in the domain
control structure.

Since touching it anyway, take the opportunity and expand
schedule_tail() in the only two places invoking it, allowing the macro
to be dropped.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>