xend: Fix error message for xm trigger command.

When I tested xm trigger command with a wrong argument, I saw the
following error message.

# xm trigger vm1 xyz
Error: __init__() takes exactly 2 arguments (3 given)
Usage: xm trigger <Domain> <nmi|reset|init> [<VCPU>]

Send a trigger to a domain.

This patch fixes the error message as follows.

# xm trigger vm1 xyz
Error: Invalid trigger: xyz
Usage: xm trigger <Domain> <nmi|reset|init> [<VCPU>]

Send a trigger to a domain.

The type of "TRIGGER_TYPE" is dictionary. domain_send_trigger()
refers to the keys of "TRIGGER_TYPE" without using keys() currently.
This patch adds keys() there.

Signed-off-by: Masaki Kanno <kanno.masaki@jp.fujitsu.com>