This patch use same expression as vif for vmx guest but has a distinguish "type=ioemu"

This patch use same expression as vif for vmx guest but has a distinguish "type=ioemu"
just like block device.
e.g. vif=['type=ioemu, mac=01:00:00:00:00:33, bridge=xenbr0']
type=ioemu to specify for vmx NIC device.
nics=4 can specify 4 vmx NIC now.
bridge is passed to device model now.
With this patch, the error
"Error: Device 0 (vif) could not be connected."
will not happen for creating vmx guest
currently, it happens on some linux distribution.

Signed-off-by: Xiaofeng Ling <xiaofeng.ling@intel.com>