lvmetad: Fix autoactivation for MDA-less PVs.

lvmetad: Fix autoactivation for MDA-less PVs.

Calling pvscan --cache with -aay on a PV without an MDA would spuriously fail
with an internal error, because of an incorrect assumption that a parsed VG
structure was always available. This is not true and the autoactivation handler
needs to call vg_read to obtain metadata in cases where the PV had no MDAs to
parse. Therefore, we pass vgid into the handler instead of the (possibly NULL)
VG coming from the PV's MDA.