vconn: Replace minimum version with bitmap of allowed versions.

vconn: Replace minimum version with bitmap of allowed versions.

This is in preparation for allowing the range of allowed OpenFlow versions
to be configured.

As part of this change pvconn_open() is now paramatised over the allowed
versions.  this is to avoid avoids needing to provide version information
as a parameter to pvconn_accept().  This will in turn avoid the need to
pass version information to connmgr_run().

Signed-off-by: Simon Horman <horms@verge.net.au>
[blp@nicira.com simplified slightly and generalize log messages]
Signed-off-by: Ben Pfaff <blp@nicira.com>