Use FIDL for QEMU pipe stream on Fuchsia.

This is needed as POSIX IO support on Fuchsia is being
deprecated. Benchmarks also show that FIDL API is ~ 25X
faster for large transfers to host so this will result
in a performance improvement.

Bug: 111137294
Test: none
Change-Id: I4538adb5075a25a2d099332b6f3eae42eab717e5
Merged-In: Iedda195ef0931c1f561f4b60ab811d968a83c0e8