Improve error message for ssh

This message was always returning "SSHPowerDriver unknown virt_type
(None)", it was suppose to at least shows the inputed virt_type value
by the user. This patch fix it making the message more user-friendly by
showing what was the valued inputed by the user + the supported values
by that module.

Change-Id: I92a78dfe065d9a38fab718bbc710b1adf933ee41
Merge "Improve error message for ssh"