In https://review.openstack.org/243098 nova_virt_type auto-detection
was implemented, but the default nova_virt_type value of kvm was left
in place, resulting in the auto-detection never happening.
This patch implements the auto-detection if nova-virt_type is not set
and removes the default value for nova-virt_type.
Change-Id: Ic1c8f042bc6bbee542373d335f338866efa06dc7