libxc/pvh: set default MTRR type to write-back

libxc/pvh: set default MTRR type to write-back

And enable MTRR. This allows to provide a sane initial MTRR state for
PVH DomUs. This will have to be expanded when pci-passthrough support
is added to PVH guests, so that MMIO regions of devices are set as
UC.

Note that initial MTRR setup is done by hvmloader for HVM guests,
that's not used by PVH guests.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>