system: vm: Support cross-compilation.

system: vm: Support cross-compilation.

* gnu/system.scm (system-linux-image-file-name): Add support for cross-built
systems. Remove system argument that was ignored,
(operating-system-kernel-file): adapt by removing ignored os argument.
* gnu/system/vm.scm (expression->derivation-in-linux-vm): Add target
argument and turn inputs into native-inputs. Pass target to qemu-command
and gexp->derivation calls.
(iso9660-image): Add target argument and pass it to
expression->derivation-in-linux-vm. Remove qemu from inputs as it
is not necessary.
(qemu-image): Add target argument, also remove qemu from inputs. Pass
target argument to expression->derivation-in-linux-vm call.