public/xen.h: add flags field to vcpu_time_info

public/xen.h: add flags field to vcpu_time_info

This field has two possible flags (as of latest pvclock ABI
shared with KVM).

flags: bits in this field indicate extended capabilities
coordinated between the guest and the hypervisor.  Specifically
on KVM, availability of specific flags has to be checked in
0x40000001 cpuid leaf. On Xen, we don't have that but we can
still check some of the flags after registering the time info
page since a force_update_vcpu_system_time is performed.

Current flags are:

 flag bit   | cpuid bit    | meaning
-------------------------------------------------------------
            |              | time measures taken across
     0      |      24      | multiple cpus are guaranteed to
            |              | be monotonic
-------------------------------------------------------------
            |              | guest vcpu has been paused by
     1      |     N/A      | the host
            |              |
-------------------------------------------------------------

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Add XEN_ prefixes to new #define-s. Make structure layout change
dependent upon __XEN_INTERFACE_VERSION__ (intentionally comparing to
4.6, as we may want to backport this at least there).

Signed-off-by: Jan Beulich <jbeulich@suse.com>