The added net-update command in virsh should allow an dhcp-host update without restarting the virtual network (I have not tested it yet).
net-update network command section xml [--parent-index index] [[--live] [--config] | [--current]]
Update the given section of an existing network definition, with the changes optionally taking effect immediately, without needing to destroy and re-start the
command is one of "add-first", "add-last", "add" (a synonym for add-last), "delete", or "modify".
section is one of "bridge", "domain", "ip", "ip-dhcp-host", "ip-dhcp-range", "forward", "forward-interface", "forward-pf", "portgroup", "dns-host", "dns-txt", or
"dns-srv", each section being named by a concatenation of the xml element hierarchy leading to the element being changed. For example, "ip-dhcp-host" will change a
<host> element that is contained inside a <dhcp> element inside an <ip> element of the network.
xml is either the text of a complete xml element of the type being changed (e.g. "<host mac="00:11:22:33:44:55' ip='188.8.131.52'/>", or the name of a file that contains
a complete xml element. Disambiguation is done by looking at the first character of the provided text - if the first character is "<", it is xml text, if the first
character is not "<", it is the name of a file that contains the xml text to be used.
The --parent-index option is used to specify which of several parent elements the requested element is in (0-based). For example, a dhcp <host> element could be in
any one of multiple <ip> elements in the network; if a parent-index isn't provided, the "most appropriate" <ip> element will be selected (usually the only one that
already has a <dhcp> element), but if --parent-index is given, that particular instance of <ip> will get the modification.
If --live is specified, affect a running network. If --config is specified, affect the next startup of a persistent network. If --current is specified, affect the
current network state. Both --live and --config flags may be given, but --current is exclusive. Not specifying any flag is the same as specifying --current.