crimson/os/seastore: use configured device type to select segment manager

In get_segment_manager(), trust the user-specified device type rather
than probing the device for ZNS zones. This simplifies the
device type selection. More important:  the change avoids opening a block
file which was not yet created by the mkfs (we start() seastore then
call mkfs. but starting seastore requires creating the right segment
manager. And, currently, we probe the not-yet-created block file in mkfs()
when trying to create it.)

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>
Co-authored-by: Kefu Chai <k.chai@proxmox.com>