Set MAP_POPULATE on guest mmap

This directs the Linux kernel to set up page tables eagerly instead
of the default lazy behaviour, and shaves about 2 ms off the FreeBSD
kernel boot time.

Signed-off-by: Colin Percival <cperciva@freebsd.org>

Author: cperciva

src/vm-memory/src/lib.rs

@@ -120,7 +120,7 @@ pub fn create_guest_memory(
 
     for region in regions {
         let flags = match region.0 {
-            None => libc::MAP_NORESERVE | libc::MAP_PRIVATE | libc::MAP_ANONYMOUS,
+            None => libc::MAP_NORESERVE | libc::MAP_POPULATE | libc::MAP_PRIVATE | libc::MAP_ANONYMOUS,
             Some(_) => libc::MAP_NORESERVE | libc::MAP_PRIVATE,
         };