gnu: Add qemu-minimal.

* gnu/packages/qemu.scm (qemu): Exchange with former QEMU-HEADLESS.
(qemu-headless): Rename to...
(qemu-minimal): ... this.  New variable.  Pass --target-list.
* gnu/packages/debug.scm (qemu-2.3.0): Inherit from QEMU-MINIMAL.
* gnu/packages/grub.scm (qemu-for-tests): Likewise.
* gnu/system/vm.scm (expression->derivation-in-linux-vm, qemu-image):
Update to name change.