vm: Allow images to be marked as non-substitutable.

* gnu/system/vm.scm (expression->derivation-in-linux-vm): Add
 #:substitutable? parameter.  Pass it to 'gexp->derivation'.
(qemu-image): Add #:substitutable? and pass it to
'expression->derivation-in-linux-vm'.
(system-disk-image): Add #:substitutable? and pass it to 'qemu-image'.