VMware: add serial port device

Add VirtualSerialPort device on newly created instances which is
backed with a file (console.log) on the datastore.
This patch will also allow to implement get_console_output() for
the VMware driver.

Change-Id: Ieeab24b7818f6d818c3922e4f5d9863a337e9a34
Closes-Bug: #1294098