* gnu/system/vm.scm (system-qemu-image/shared-store): Add
#:disk-image-size and #:full-boot? parameters and honor them. Pass
'#:copy-inputs? full-boot?', and change #:inputs argument.
* guix/scripts/system.scm (system-derivation-for-action): Pass
#:disk-image-size to 'system-qemu-image/shared-store'.
* doc/guix.texi (Invoking guix system): Mention use of '--image-size' in
conjunction with '--full-boot'.