docs/dev/libvirt-howto: Fix "firewalld-cmd" -> "firewall-cmd" typo
docs/dev/libvirt-howto: Fix "firewalld-cmd" -> "firewall-cmd" typo

The typo is from af6d904c (fixing cmd and typo, 2018-09-20, #293),
which was itself fixing typos from 21ef0d4f (adding details regarding
using of firewalld instead of iptables, 2018-09-19, #284).

derekhiggins/installer,derekhiggins/installer