ovs-vsctl: Allow referring to rows whose names are UUIDs by those names.

Every row in the database has a UUID, generated by the database server.
Rows in most tables also have a user-assigned name (e.g. a bridge or port
or interface name).  The ovs-vsctl database commands (e.g. "set", "get",
"list", ...) accept both UUIDs and names, but until now if a command's
argument had the form of a UUID, then it had to be the database-assigned
UUID for a row; that is, ovs-vsctl did not check whether it was the name
of a row.  This commit changes that: a UUID argument to a database command
is now first checked against database UUIDs then, if it is not a database
UUID, it is checked as the name of a row.

This is prompted by Windows integration with OpenStack, which uses UUIDs
as port names.

CC: Nithin Raju <nithin@vmware.com>
Requested-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Gurucharan Shetty <gshetty@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>