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>

     1.1 --- a/tools/ioemu/target-i386-dm/qemu-ifup	Sat Dec 29 17:57:47 2007 +0000
     1.2 +++ b/tools/ioemu/target-i386-dm/qemu-ifup	Tue Jan 08 09:40:33 2008 +0000
     1.3 @@ -34,4 +34,4 @@ then
     1.4  fi
     1.5  
     1.6  ifconfig $1 0.0.0.0 up
     1.7 -brctl addif $bridge $1
     1.8 +brctl addif $bridge $1 || true