qemu-arm: Enable NVMe for distro boot

We already support the NVMe commands and PCIe backend in the QEMU target,
so let's make it easy for anyone to consume them and enable NVMe distro
boot along the way!

With this patch, I can put an NVMe backed disk image into my QEMU VM and
have it automatically load a UEFI target blob.

Signed-off-by: Alexander Graf <agraf@csgraf.de>
Reviewed-by: Mark Kettenis <kettenis@openbsd.org>