[fdio] Add the ability to create an fd from a zxio

This CL creates the ability to make a file descriptor from an arbitrary
zxio object. This CL then uses this ability in chromeos-disk-setup to
replace its use of fdio_vmo_fd.

Test: chromeos-disk-setup
Change-Id: I51eca7ba9f326d44df97c8993c082b09a113634f