v2v: Fix kernel detection when multiple kernels are installed (RHBZ#1161250).

Previously we used to try to find the 'vmlinuz' file by running 'rpm
-ql kernel' and looking for any file called 'vmlinuz-*'.

If there were multiple 'kernel' packages installed, the rpm command
would list files from all of them, resulting in a random 'vmlinuz-*'
being chosen, not necessarily the one corresponding to the kernel
package we were looking at.

Use 'rpm -ql kernel-<VERSION>-<RELEASE>' instead so that we only look
for files in the right kernel package.

Thanks: James Mighion
v2v: Warn if a kernel-* package is found containing no files.