Instead of invoking select_watchdog_action from both HMP and command line,
go directly from HMP to QMP and use QemuOpts as the intermediary for the
command line.

This makes -watchdog-action explicitly a shortcut for "-action watchdog",
so that "-watchdog-action" and "-action watchdog" override each other
based on the position on the command line; previously, "-action watchdog"
always won.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>