vm: Support initialization of the store DB when the store is shared.

* gnu/system/vm.scm (qemu-image): Rename #:inputs-to-copy to #:inputs,
  and #:initialize-store? to #:register-closures?.  Add #:copy-inputs?.
  Adjust build gexp accordingly.
  (system-qemu-image): Remove #:initialize-store? argument and add
  #:copy-inputs?.
  (system-qemu-image/shared-store): Add #:inputs, #:register-closures?,
  and #:copy-inputs? arguments.
* guix/build/vm.scm (register-closure): New procedure.
  (MS_BIND): New variable.
  (initialize-hard-disk): Rename #:initialize-store? to
  #:register-closures?, #:closures-to-copy to #:closures, and add
  #:copy-closures?.
  Add 'target-directory' and 'target-store' variables.
  Call 'populate-store' only when COPY-CLOSURES?.
  Bind-mount the store to TARGET-STORE when REGISTER-CLOSURES? and not
  COPY-CLOSURES?.  Add call to 'register-closure'.