cam: Add xpt_gdev_type() and use it instead of many copies of same

Add a convenience wrapper to XPT_GDEV_TYPE in the same way we have one
for XPT_PATH_INQ. The changes from PRIORITY_NORMAL to PRIORITY_NONE are
intentional because this isn't a queued CCB. Please note: we have
several places still that construct a XPT_GDEV_TYPE message by
overwriting a CCB that happens to be laying around. I've not used this
method, by and large, in those places since I didn't want to risk
upsetting allocation flags that might be present (since we use a specail
allocator for some CCB types in *_da.c).

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D51216