bhyve: increase number of supported consoles to 4

bhyve: increase number of supported consoles to 4

Recent versions of bhyve support 4 com ports instead of just 2. Thus,
allow to use 4 console devices.

Also, there was a bug previously because the condition was
"if (chr->target.port > 2)", but as target.port start
with 0 and "com" ports start with 1, this condition allows com3 to be
used.

As bhyve supports 4 com ports already long enough, and all supported
FreeBSD versions include this capability, do not introduce driver
capability for that.

Add a couple of tests for that:

 - A domain that uses 4 serials, 2 of type 'nmdm'
   and the other 2 of type 'tcp'
 - A domain that uses unsupported port, such as target.port=4 which
   translates into com5.

Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>