A channel with id DEVMGR_LAUNCHER_DEVFS_ROOT_HND will have the devfs
server attached to it.  This will let tests interact with devfs.

Test: Booted and ran a test program that successfully reported
the unprivileged devmgr's devfs being populated.

Change-Id: Ic65dafd6b5197e82e7b7f345766b6aee281e00d8