vm: Estimate the disk size by default.

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.