Fix config file parsing for VMX domains.

Fix config file parsing for VMX domains.

If we define "vif" in the config file, image.py will raise error "vmx:
missing vbd configuration". The reason is "vif" is dealt with as a "vbd"
device.

This patch fixes this issue by dealing with "vbd" and "vif " separately,
removing "macaddr" arg and parsing mac address from "vif" instead.

Also, the vbd doesn't have to be a file anymore, but can be a physical
disk partition.

Signed-off-by: Yunfeng Zhao <yunfeng.zhao@intel.com>
Signed-off-by: Arun Sharma <arun.sharma@intel.com>