Implement cluster verify checks for wrong PV names

Since ':' is not a valid character in PV names (for the way Ganeti uses
LVM), we need to check this and warn the user. This patch adds a new
NV_PVLIST cluster verify check and verifies the PV names returned from
the nodes.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>