x86/virt/xen: Use guest_late_init to detect Xen PVH guest

In case we are booted via the default boot entry by a generic loader
like grub or OVMF it is necessary to distinguish between a HVM guest
with a device model supporting legacy devices and a PVH guest without
device model.

PVH guests will always have x86_platform.legacy.no_vga set and
x86_platform.legacy.rtc cleared, while both won't be true for HVM
guests.

Test for both conditions in the guest_late_init hook and set xen_pvh
to true if they are met.

Move some of the early PVH initializations to the new hook in order
to avoid duplicated code.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: boris.ostrovsky@oracle.com
Cc: xen-devel@lists.xenproject.org
Link: http://lkml.kernel.org/r/20171109132739.23465-6-jgross@suse.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>