Allow build-support RPM VMs to be booted with their native kernels.
Also create (/fs)/run in build-support VM bootstrap code.
VMs: Include store paths used in an initrd in its closure.
VMs: Restore CIFS/samba as a fallback for kernels with broken 9p/virtfs.
VMs: When mounting w/ CIFS, do not assume 9P /nix/store was mounted.
VMs: Make insmod failures non-fatal.
VMs: Allow custom phases in RPM builds, make configurePhase meaningful.
VMs: Use devtmpfs for /dev when available.
VMs: Use findTarballs when building RPMs.
build-support: Simplify findTarballs.