When I was playing with Solaris 9 under qemu, I needed to install driver for network adapter (the default ne2k_pci). Rtl8139 neither other options doesn't seem to work not even in linux. Lspci says nothing when -net nic,model=<anything but ne2k_pci> is used in linux, the same result for prtconf in Solaris.
To get it right I've tried nei driver without success. Next thing I was told to test by google (and maybe Solaris HCL) was ni driver from this page. And it works, despite it's marked as beta. (You would propbably need to add_drv -i '"pci10ec,8029"' ni to add the driver, touch /reconfigure to tell Solaris to recheck hardware and reboot. Remember to create /etc/hostname.ni0 and /etc/dhcp.ni0 if you want to use dhcp client on this adapter (as you want to on qemu).
Just a note, use ls more than I. I installed gcc and make and after that I figured out, that there is already a binary driver in i386 (alias obj) dir. So you don't need to compile anything, if you don't want to.