hmp: Fix definition of command quit

The command handler is a union of two function types.  If
cmd->user_print is set, handle_user_command() calls
cmd->mhandler.cmd_new(), else cmd->mhandler.cmd().

Command definitions must therefore either set both user_print() and
mhandler.cmd_new(), or only mhandler.cmd().

quit's sets user_print and mhandler.cmd().  handle_user_command()
calls hmp_quit() through mhandler.cmd_new() rather than
mhandler.cmd(), i.e. through a function pointer with a different type.
Broken in commit 7a7f325e, v1.0.

Works in practice because hmp_quit() doesn't use its arguments, and
handle_user_command() ignores its function value.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>