Construct the memory source from a file descriptor

vmie_mem could only be built from a path. Add a second input: an
already-open file descriptor, dup()'d internally (borrowed - the
caller's fd stays valid, core owns and closes its copy).

  - vmie_mem_from_fd / vmie_mem_from_fd_segs   (read-write, as the path
    constructors: PROT_RW, MAP_SHARED)
  - vmie_mem_from_ro_fd / vmie_mem_from_ro_fd_segs   (read-only: map
    PROT_READ, mark the source ro; gpa_write/gva_write return -1, every
    read path is unchanged; accepts an O_RDONLY fd)
  - vmie_win32_open_fd   (the win32 context over an fd backing file)

Factor the mmap/validate tail and the single-low segment map out of the
path constructors into shared helpers, so the path and fd inputs go
through one mmap site and one map builder each.

src/engine/win32/host.c

@@ -21,6 +21,19 @@ vmie_win32* vmie_win32_open(const char* ram_path, uint64_t low) {
     return v;
 }
 
+__attribute__((cold))
+vmie_win32* vmie_win32_open_fd(int fd, uint64_t low) {
+    vmie_win32* v = calloc(1, sizeof *v);
+    if (!v) {
+        return NULL;
+    }
+    if (gpa_from_fd(&v->mem, fd, low)) {
+        free(v);
+        return NULL;
+    }
+    return v;
+}
+
 __attribute__((cold))
 void vmie_win32_close(vmie_win32* v) {
     if (!v) {