KVM: MMU: Update shadow ptes on partial guest pte writes

A guest partial guest pte write will leave shadow_trap_nonpresent_pte
in spte, which generates a vmexit at the next guest access through that pte.

This patch improves this by reading the full guest pte in advance and thus
being able to update the spte and eliminate the vmexit.

This helps pae guests which use two 32-bit writes to set a single 64-bit pte.

[truncation fix by Eric]

Signed-off-by: Yaozu (Eddie) Dong <eddie.dong@intel.com>
Signed-off-by: Feng (Eric) Liu <eric.e.liu@intel.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>