x86/PV: support data breakpoint extension registers

x86/PV: support data breakpoint extension registers

Introducing an extension to XEN_DOMCTL_[gs]et_ext_vcpucontext similar
to the generic MSR save/restore logic recently added for HVM.

This also moves some debug register related declarations/definition to
the header intended for these.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Aravind Gopalakrishnan<aravind.gopalakrishnan@amd.com>
Reviewed-by: Aravind Gopalakrishnan<aravind.gopalakrishnan@amd.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>