Fix save/restore of guest PAT table in HAP paging mode.

Fix save/restore of guest PAT table in HAP paging mode.

HAP paging mode guests use direct MSR read/write into the VMCS/VMCB
for the guest PAT table, while the current save/restore code was
accessing only the pat_cr field in hvm_vcpu, used when intercepting
the MSR mostly in shadow mode (the Intel scenario is a bit more
complicated).  This patch fixes this issue creating a new couple of
hvm_funcs, get/set_guest_pat, that access the right PAT table based on
the paging mode and guest configuration.

Signed-off-by: Gianluca Guida <gianluca.guida@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Keir Fraser <keir@xen.org>