pve7to8: add check for systemd-boot presence where needed

pve7to8: add check for systemd-boot presence where needed

since the package won't get installed for systems upgraded from 7 to 8
we warn users who need systemd-boot - to be able to initialize new
ESPs - that they need to install it

The check for package installation is based on existance of the
changelog, since the package information used in pve7to8 comes from
the API-modules, which limit it to the pve-relevant packages.

tested in VMs with uefi and legacy mode, with existing
proxmox-boot-uuids both with and w/o systemd-boot being installed

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>