vm: Estimate the disk size by default.

* gnu/build/vm.scm (estimated-partition-size): New procedure.
* gnu/system/vm.scm (expression->derivation-in-linux-vm):
Change #:disk-image-size default to 'guess.
[builder]: When DISK-IMAGE-SIZE is 'guess, use
'estimated-partition-size' and compute and estimate of the image size.
(qemu-image): Likewise.
* guix/build/store-copy.scm (file-size, closure-size): New procedures.
* guix/scripts/system.scm (%default-options): Change 'image-size' to
'guess.
* doc/guix.texi (Building the Installation Image): Remove '--image-size'
flag from example.
(Invoking guix system): Document the image size estimate.

gnu/build/vm.scm

@@ -46,6 +46,7 @@
             partition-flags
             partition-initializer
 
+            estimated-partition-size
             root-partition-initializer
             initialize-partition-table
             initialize-hard-disk))
@@ -150,6 +151,12 @@ the #:references-graphs parameter of 'derivation'."
   (flags       partition-flags (default '()))
   (initializer partition-initializer (default (const #t))))
 
+(define (estimated-partition-size graphs)
+  "Return the estimated size of a partition that can store the store items
+given by GRAPHS, a list of file names produced by #:references-graphs."
+  ;; Simply add a 20% overhead.
+  (round (* 1.2 (closure-size graphs))))
+
 (define (fold2 proc seed1 seed2 lst)              ;TODO: factorize
   "Like `fold', but with a single list and two seeds."
   (let loop ((result1 seed1)