util: virFirewallDGetPolicies: gracefully handle older firewalld

util: virFirewallDGetPolicies: gracefully handle older firewalld

If the running firewalld doesn't support getPolicies() then we fallback
to the "libvirt" zone. Throwing an error log is excessive since we
gracefully fallback.

Avoids these logs:

error : virGDBusCallMethod:242 : error from service: \
GDBus.Error:org.freedesktop.DBus.Error.UnknownMethod

Fixes: ab56f84976e0 ("util: add virFirewallDGetPolicies()")
Signed-off-by: Eric Garver <eric@garver.life>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>