SW: add qemu-system-x86 as underlying dependency for qemu-kvm

SW: add qemu-system-x86 as underlying dependency for qemu-kvm

Otherwise deborphan tries to remove qemu-system-x86 package as it
wasn't installed explicitely, then resulting in:

| The following packages were automatically installed and are no longer required:
|   ipxe-qemu libcaca0 libcacard0 libfdt1 libsdl1.2debian libspice-server1
|   libusbredirparser1 libvdeplug2 libxen-4.8 libxenstore3.0 libyajl2
|   qemu-system-common qemu-system-x86 seabios
| Use 'sudo apt autoremove' to remove them.
| The following packages will be REMOVED:
|   qemu-kvm*

which is clearly unwanted behavior if we want to have qemu/kvm
available.