vmspawn: reject --bind-volume= duplicates at parse time (#41961)

bind_volume_parse() does not look at peers, so passing the same
PROVIDER:VOLUME twice on the command line silently produces two parsed
entries in arg_bind_volumes. vmspawn_bind_volume_acquire() then builds
two DriveInfo with identical d->id ("<provider>:<volume>"). At boot,
bridge_register_drive() puts d->id into the b->block_devices hashmap;
the second insert returns -EEXIST and the user sees a bare "File exists"
with no context for which volume is responsible.

Reject the collision at the parse site with a linear scan over the
existing array — n_items is small (one entry per --bind-volume on the
command line), and a clear error message naming the offending volume is
much more useful than the late EEXIST from the QMP setup loop.