CLOUDSTACK-6212: Let vm_instance table's ID field be unique and auto_inc...

...rement

`vm_instance` table's id column is unique but does not auto_increment like other
resource tables. The fix simply alter the table's id column to have it
auto_increment which may avoid id conflicts in a multi-master mysql setup.

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>