lguest: don't look in console features to find emerg_wr.

The 1.0 spec clearly states that you must set the ACKNOWLEDGE and
DRIVER status bits before accessing the feature bits.  This is a
problem for the early console code, which doesn't really want to
acknowledge the device (the spec specifically excepts writing to the
console's emerg_wr from the usual ordering constrains).

Instead, we check that the *size* of the device configuration is
sufficient to hold emerg_wr: at worst (if the device doesn't support
the VIRTIO_CONSOLE_F_EMERG_WRITE feature), it will ignore the
writes.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>