target/i386: move paging mode constants from SVM to cpu.h

We will reuse the page walker for both SVM and regular accesses.  To do
so we will build a function that receives the currently active paging
mode; start by including in cpu.h the constants and the function to go
from cr4/hflags/efer to the paging mode.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>