collectd: fix a floating dependence on libvirt

Even though the PACKAGECONFIG of libvirt is used to fix the floating
dependence, but enable-libvirt/disable-libvirt become useless after
collectd upgrade, and libvirt is checked unconditionally, and lead to
a floating dependence on libvirt.

check if libvirt is available only when a user wants to use libvirt
to fix this issue

Signed-off-by: Roy Li <rongqing.li@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>