Only attach to a GPT partition if it has the GPT_ENT_TYPE_FREEBSD type.
Only attach to a GPT partition if it has the GPT_ENT_TYPE_FREEBSD type.

XXX: This only works currently with GEOM_GPT which only exists in 6.x.
XXX: I didn't add 'mbroffset' support for a GPT partition holding a BSD
label as I'm not sure if they use relative or absolute offsets.

MFC after:	3 days

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase