This patch adds an optional attribute to the <bootp> tag, that
allows to specify a TFTP server address other than the address of
the DHCP server itself.

This can be used to forward the BOOTP settings of the host down to the
guest.  This is something that configurations such as Xen's default
network achieve naturally, but must be done manually for NAT.

* docs/formatnetwork.html.in: Document new attribute.
* docs/schemas/network.rng: Add it to schema.
* src/conf/network_conf.h: Add it to struct.
* src/conf/network_conf.c: Add it to parser and pretty printer.
* src/network/bridge_driver.c: Put it in the dnsmasq command line.
* tests/networkxml2xmlin/netboot-proxy-network.xml
  tests/networkxml2xmlout/netboot-proxy-network.xml
  tests/networkxml2xmltest.c: add new tests