qemu-options: Bail out on unsupported options instead of silently ignoring them

qemu-options: Bail out on unsupported options instead of silently ignoring them

The dangling remainder of the -tdf option revealed a deficiency in our
option parsing: Options that have been declared, but are not supported
in the switch-case statement in vl.c and not handled in the OS-specifc
os_parse_cmd_args() functions are currently silently ignored. We should
rather tell the users that they specified something that we can not
handle, so let's print an error message and exit instead.

Reported-by: Markus Armbruster <[email protected]>
Signed-off-by: Thomas Huth <[email protected]>
Message-Id: <1525453270[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>