xen/pvh: bootup and setup (E820) related changes.

xen/pvh: bootup and setup (E820) related changes.

In the bootup code for PVH we can trap cpuid via vmexit, so don't
need to use emulated prefix call. We also check for vector callback
early on, as it is a required feature. PVH also runs at default kernel
IOPL.

In setup.c which deals with E820, in xen_add_extra_mem() we can skip
updating P2M as it's managed by Xen. PVH maps the entire IO space,
but only RAM pages need to be repopulated.

Finally, pure PV settings are moved to a separate function that are
only called for pure PV, ie, pv with pvmmu.

Signed-off-by: Mukesh Rathor <mukesh.rathor@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>