Use package from libvirt flake

[phip1611]

Since Jan 28st, we build the libvirt package directly in the libvirt flake [0]. To simplify things, we should refactor the usage here.

An early attempt was made in #96. In the end, we will need something like this:

# For quicker rebuilds, which we experience quite often during development
            # and prototyping,, we remove all unneeded functionality.
            libvirt-chv-testsuite = libvirt-chv.packages.libvirt-debugoptimized.overrideAttrs (old: {
              # Reduce files needed to compile. We cut the build-time in half.
              mesonFlags = old.mesonFlags ++ [
                # Disabling tests: 1500 -> 1200
                "-Dtests=disabled"
                "-Dexpensive_tests=disabled"
                # Disabling docs: 1200 -> 800
                "-Ddocs=disabled"
                # Disabling unneeded backends: 800 -> 685
                "-Ddriver_ch=enabled"
                "-Ddriver_qemu=disabled"
                "-Ddriver_bhyve=disabled"
                "-Ddriver_esx=disabled"
                "-Ddriver_hyperv=disabled"
                "-Ddriver_libxl=disabled"
                "-Ddriver_lxc=disabled"
                "-Ddriver_openvz=disabled"
                "-Ddriver_secrets=disabled"
                "-Ddriver_vbox=disabled"
                "-Ddriver_vmware=disabled"
                "-Ddriver_vz=disabled"
                # Disabling unneeded backends: 685 -> 609
                "-Dstorage_dir=disabled"
                "-Dstorage_disk=disabled"
                "-Dstorage_fs=enabled" # for netfs
                "-Dstorage_gluster=disabled"
                "-Dstorage_iscsi=disabled"
                "-Dstorage_iscsi_direct=disabled"
                "-Dstorage_lvm=disabled"
                "-Dstorage_mpath=disabled"
                "-Dstorage_rbd=disabled"
                "-Dstorage_scsi=disabled"
                "-Dstorage_vstorage=disabled"
                "-Dstorage_zfs=disabled"
                "-Dapparmor=disabled"
                "-Dwireshark_dissector=disabled"
                "-Dselinux=disabled"
                "-Dsecdriver_apparmor=disabled"
                "-Dsecdriver_selinux=disabled"
                "-Db_sanitize=leak"
                "-Db_sanitize=address,undefined"
                # Enabling the sanitizers has led to warnings about inlining macro
                # generated cleanup methods of the glib which spam the build log.
                # Ignoring and suppressing the warnings seems like the only option.
                # "warning: inlining failed in call to 'glib_autoptr_cleanup_virNetlinkMsg': call is unlikely and code size would grow [-Winline]"
                "-Dc_args=-Wno-inline"
              ];

[0] https://gitlab.cyberus-technology.de/cyberus/cloud/libvirt/-/merge_requests/53