Here is a patch that sets the hostname in the DHCP response generated
by qemu's user-net DHCP server, and adds a new -hostname command line
option to specify the value.

If the guest OS is configured properly, the value received in the DHCP
response is automatically used to set the machine's hostname. (In Red
Hat/Fedora Linuxes, this happens if HOSTNAME is not hardcoded in
/etc/sysconfig/network.)

Feedback welcome.

--Ed