Comment 4 for bug 1495895

Have qemu-system-common and qemu-utils depend on qemu-block-extra otherwise the packaging change changes expected behavior; namely before the packaging change users could install qemu-system-x86 (and others arches) and expect to use curl or rbd as a qemu block backend. After the packaging change users now are suggested to install qemu-block-extra but it's not immediately clear that qemu itself has missing features now due to the missing libraries. This breaks charms and other scripts which have expected qemu to have specific features present. Resolve this by having a dependency on the extra package which contains the block function.