pvscan: warn about /dev/sda excluded by devices file

pvscan: warn about /dev/sda excluded by devices file

In most installations, /dev/sda* or /dev/vda* should be included
in system.devices because the root, home, etc LVs are usually on
sda or vda.

Add a special case warning when a pvscan autoactivation command
sees that /dev/sda* or /dev/vda* are excluded by system.devices,
either not listed or having a different device id.