CA-120950: new implementation of preemptible hypercalls

Use a per-CPU variable to mark hypercalls (using
xen_preemptible_hcall_begin() and xen_preemptible_hcall_end() helpers).

Call preempt_schedule_irq() from the correct place, specifically when
we're running on the non-irq stack.
CA-120950: fix broken xen trace patch

Note: When using guilt push -f with a patch that adds new files, make
sure to git add the files before guilt refresh'ing the patch.
CA-120950: add drivers/xen/preempt.c to patch
CP-7811: base Requires and Conflicts on CentOS 6 specfile
CP-7811: integration with new-kernel-pkg.
CP-7811: drop dependency on dracut-kernel, it causes our installer to balloon in size