libxl: change default HVM emulated network card to rtl8139

libxl: change default HVM emulated network card to rtl8139

xend uses rtl8139, and we want xl to be compatible with xm.  Some
older operating systems don't have e1000 drivers, and we want widest
compatibility rather than best performance (people who want good
performance are best advised to use PV-on-HVM drivers).

We'll probably switch to a new default when switching to upstream
qemu, in the Xen 4.2 release cycle.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>