Built-in DHCP server: SiAddr

From https://gitlab.com/qemu-project/qemu/-/issues/336#note_578823766 :

“ The DHCP server built into qemu for user network is quite handy for osdev, but currently lacks a way to set the SiAddr field in the responses sent out, only the DHCP option 66 can be configured with tftp-server-name=foo. Since the included iPXE firmware does not use that, that's not as useful as it could be.

I propose adding an argument siaddr (the BOOTP field name to avoid confusions with tftp-server-name) which accepts an IPv4 that is sent out in that field.

I'll probably be able to make the changes myself, but want other opinions first :) ”