qapi: convert BlockdevOptions to use enum discriminator

After this patch, hidden enum type BlockdevOptionsKind will not
be generated, and other API can use enum BlockdevDriver.

Signed-off-by: Wenchao Xia <wenchaoqemu@gmail.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

qapi-schema.json

@@ -4248,6 +4248,18 @@
             '*direct': 'bool',
             '*no-flush': 'bool' } }
 
+##
+# @BlockdevDriver
+#
+# Drivers that are supported in block device operations.
+#
+# Since: 2.0
+##
+{ 'enum': 'BlockdevDriver',
+  'data': [ 'file', 'http', 'https', 'ftp', 'ftps', 'tftp', 'vvfat', 'blkdebug',
+            'blkverify', 'bochs', 'cloop', 'cow', 'dmg', 'parallels', 'qcow',
+            'qcow2', 'qed', 'raw', 'vdi', 'vhdx', 'vmdk', 'vpc', 'quorum' ] }
+
 ##
 # @BlockdevOptionsBase
 #
@@ -4272,7 +4284,7 @@
 # Since: 1.7
 ##
 { 'type': 'BlockdevOptionsBase',
-  'data': { 'driver': 'str',
+  'data': { 'driver': 'BlockdevDriver',
             '*id': 'str',
             '*node-name': 'str',
             '*discard': 'BlockdevDiscardOptions',