On a multi-device Btrfs filesystem `grub-probe --target=device /` prints
one device per line (e.g. /dev/vda3 and /dev/vdb). The whole multi-line
result was then passed to `grub-probe --device "${root_device}"
--target=fs_uuid`, which cannot handle several devices at once and
returns an empty string. The blkid fallback fails for the same reason,
so grub-btrfs aborts with "Cannot determine UUID".
Pipe the device probe through `head -n1` and keep only the first member
device. Every member of a Btrfs pool reports the same pool-wide fs UUID,
so the first device is sufficient. For single-device setups `head -n1`
is a no-op, so existing configurations are unaffected.
The same one-line-per-device behaviour applies to a /boot that lives on
a multi-device Btrfs, so the identical guard is added to boot_device.
Closes#412
Refs #425
