virconf: Introduce VIR_CONF_ULONG

virconf: Introduce VIR_CONF_ULONG

https://bugzilla.redhat.com/show_bug.cgi?id=1160995

In our config files users are expected to pass several integer values
for different configuration knobs. However, majority of them expect a
nonnegative number and only a few of them accept a negative number too
(notably keepalive_interval in libvirtd.conf).
Therefore, a new type to config value is introduced: VIR_CONF_ULONG
that is set whenever an integer is positive or zero. With this
approach knobs accepting VIR_CONF_LONG should accept VIR_CONF_ULONG
too.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>