pc: Define PC_COMPAT_2_[123] macros

Once we start adding compat code for pc-2.3, the usage of HW_COMPAT_2_1
in pc-*-2.2 won't be enough, as it also has to include PC_COMPAT_2_3
inside it. To ensure that, define PC_COMPAT_2_3, PC_COMPAT_2_2, and
PC_COMPAT_2_1 macros.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>