rbd: Don't limit length of parameter values

We laboriously enforce that parameter values are between one and some
arbitrary limit in length.  Only RBD_MAX_IMAGE_NAME_SIZE comes from
librbd.h, and I'm not sure it applies.  Where the other limits come
from is unclear.

Drop the length checking.  The limits librbd actually imposes must be
checked by librbd anyway.

There's one minor complication: BDRVRBDState member name is a
fixed-size array.  Depends on the length limit.  Make it a pointer to
a dynamically allocated string.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-id: 1490691368-32099-4-git-send-email-armbru@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>
