block/rbd: add support for 'mon_host', 'auth_supported' via QAPI

block/rbd: add support for 'mon_host', 'auth_supported' via QAPI

This adds support for three additional options that may be specified
by QAPI in blockdev-add:

    server: host, port
    auth method: either 'cephx' or 'none'

The "server" and "auth-supported" QAPI parameters are arrays.  To conform
with the rados API, the array items are join as a single string with a ';'
character as a delimiter when setting the configuration values.

Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>