HVM vif without bridge.

HVM vif without bridge.
When using xen without a bridge but NAT or routing, HVM domains can't
boot, and qemu-dm-n.log contains:

config qemu network with xen bridge for  tap0 xenbr0
bridge xenbr0 does not exist!

That's because the qemu-ifup script always tries to add the vif to
a default-named xenbr0 bridge. On the contrary, PV domains just work
fine with the same configuration file except HVM parameters.

Signed-off-by: Samuel Thibault <samuel.thibault@citrix.com>