-watchdog is the same as -device except that it is case insensitive (and it
allows only watchdog devices of course). Now that "-device help" can list
as such the available watchdog devices, we can deprecate it.
Note that even though -watchdog tries to be case insensitive, it fails
at that: "-watchdog i6300xyz" fails with "Unknown -watchdog device",
but "-watchdog i6300ESB" also fails (when the generated -device option
is processed) with an error "'i6300ESB' is not a valid device model name".
For this reason, the documentation update does not mention the case
insensitivity of -watchdog.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>