gnt-node volumes: Fix instance names
gnt-node volumes: Fix instance names

Commit 84d7e26b changed “objects.Instance.MapLVsByN” to not just return
the LV name, but to include the volume group name (e.g.
“xenvg/d67e8700….disk0_data”). This in turn broke the mapping of volume
names in LUNodeQueryvols, stopping instance names from displayed in
“gnt-node volumes”.

This patch fixes the issue and does some cleanup.

Signed-off-by: Michael Hanselmann <68b67885ff74281219b4a4be33cc20fd45078d7c@google.com>
Reviewed-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>

ganeti/ganeti,bitemyapp/ganeti,leshchevds/ganeti,onponomarev/ganeti,onponomarev/ganeti,ganeti-github-testing/ganeti-test-1,leshchevds/ganeti,andir/ganeti,ganeti/ganeti,grnet/snf-ganeti,yiannist/ganeti,apyrgio/snf-ganeti,andir/ganeti,dimara/ganeti,yiannist/ganeti,leshchevds/ganeti,apyrgio/snf-ganeti,andir/ganeti,dimara/ganeti,ganeti/ganeti,mbakke/ganeti,mbakke/ganeti,yiannist/ganeti,bitemyapp/ganeti,apyrgio/ganeti,grnet/snf-ganeti,mbakke/ganeti,apyrgio/ganeti,ganeti-github-testing/ganeti-test-1