Add backward compatibility for kernel vm image

Add backward compatibility for kernel vm image

https://gerrit.fd.io/r/#/c/20577/ replaced kvm kernel with custom build
of generic kernel, which would be picked up with '/opt/boot/vmlinuz*'
wildcard and which wouldn't work with this code. Fix this by adding
'-kvm' to the wildcard so that the kvm image is used with this code.

Change-Id: I9e9d978ad7c980d74f88f761b884eca599dee454
Signed-off-by: juraj.linkes <juraj.linkes@pantheon.tech>