distro_bootcmd: refactor virtio to support PCI block devices

distro_bootcmd: refactor virtio to support PCI block devices

Starting libvirt v5.3.0 with QEMU 4.0.0 use of PCI is automatic
and thus storage is connected via PCI, which is not visible to
U-Boot out-of-the-box.

Refactor to do "pci enum" followed by "virtio scan" to see PCI
connected storage, and allow bootloader to load kernel and
initramfs images.

Tested with Fedora/RISCV using releases: libvirt 5.4.0 & 5.5.0,
QEMU 4.0.0 and U-Boot 2019.07 RC4.

Signed-off-by: David Abdurachmanov <david.abdurachmanov@sifive.com>