build: Add option to control building libvirtd

build: Add option to control building libvirtd

Add meson option 'libvirtd' to control building of the monolithic
libvirtd, independent from the stateful, daemon-based drivers. E.g.

meson configure -Dlibvirtd=disabled build-dir

The new option defaults to 'auto' and will be enabled when 'driver_libvirtd'
is enabled. Setting libvirtd=disabled skips building the libvirtd binary,
and related items such as config files, systemd units, etc.

The existing option 'driver_libvirtd' controls building libvirtd and all
stateful, daemon-based drivers, making it unsuitable for disabling only
the monolithic daemon. Additionally, 'driver_libvirtd' is used by the
various libvirt-ci projects to produce a minimalistic build.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>