runstate: cleanup reboot and panic actions

The possible choices for panic, reset and watchdog actions are inconsistent.

"-action panic=poweroff" should be renamed to "-action panic=shutdown"
on the command line.  This is because "-action panic=poweroff" and
"-action watchdog=poweroff" have slightly different semantics, the first
does an unorderly exit while the second goes through qemu_cleanup().  With
this change, -no-shutdown would not have to change "-action panic=pause"
"pause", just like it does not have to change the reset action.

"-action reboot=none" should be renamed to "-action reboot=reset".
This should be self explanatory, since for example "-action panic=none"
lets the guest proceed without taking any action.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>