Fixes for issues #8065 and #8075 in mysql_db and #8067 in vsphere_guest

This fixes three traceback errors:

- When creating or deleting a mysql database fails (issue #8065)
- When a named vsphere datacenter does not exist (issue #8067)
- When a vsphere guest is created without notes
- When a vsphere guest is created without a requirement to power on/off (and also without notes)

It also responds to issue #8075 by updating the mysql_db code so it matches the documentation.