runqemu: try symlinks when kernel or rootfs can't be found

runqemu: try symlinks when kernel or rootfs can't be found

If the kernel or rootfs names written to the qemuboot.conf can't
be found, try and find the symlinked variant of the filename.

This will help usability of runqemu, for example where a user
downloads an image and associated files as the symlinked names
yet the qemuboot.conf variables point to the full, non-linked,
file names.

Signed-off-by: Joshua Lock <joshua.g.lock@intel.com>