verify: use same progress as pull

verify: use same progress as pull

percentage of verified groups, interpolating based on snapshot count
within the group. in most cases, this will also be closer to 'real'
progress since added snapshots (those which will be verified) in active
backup groups will be roughly evenly distributed, while number of total
snapshots per group will be heavily skewed towards those groups which
have existed the longest, even though most of those old snapshots will
only be re-verified very infrequently.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>