qapi: convert add_client

Also fixes a few issues while there:

 1. The fd returned by monitor_get_fd() leaks in most error conditions
 2. monitor_get_fd() return value is not checked. Best case we get
    an error that is not correctly reported, worse case one of the
    functions using the fd (with value of -1) will explode
 3. A few error conditions aren't reported
 4. We now "use up" @fdname always.  Before, it was left alone for
    invalid @protocol

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>

qapi-schema.json

@@ -32,6 +32,31 @@
             'DeviceNotActive', 'DeviceNotFound', 'KVMMissingCap',
             'MigrationExpected' ] }
 
+##
+# @add_client
+#
+# Allow client connections for VNC, Spice and socket based
+# character devices to be passed in to QEMU via SCM_RIGHTS.
+#
+# @protocol: protocol name. Valid names are "vnc", "spice" or the
+#            name of a character device (eg. from -chardev id=XXXX)
+#
+# @fdname: file descriptor name previously passed via 'getfd' command
+#
+# @skipauth: #optional whether to skip authentication. Only applies
+#            to "vnc" and "spice" protocols
+#
+# @tls: #optional whether to perform TLS. Only applies to the "spice"
+#       protocol
+#
+# Returns: nothing on success.
+#
+# Since: 0.14.0
+##
+{ 'command': 'add_client',
+  'data': { 'protocol': 'str', 'fdname': 'str', '*skipauth': 'bool',
+            '*tls': 'bool' } }
+
 ##
 # @NameInfo:
 #