runqemu: Move virtio RNG to machine configuration

runqemu: Move virtio RNG to machine configuration

Not all QEMU machines (outside of those available in OE-Core) are
capable of using the virtio-rng-pci device due to various machine models
not having a pci/virtio bus. This makes it such that the use of the
'-device virtio-rng-pci' flag to QEMU is machine specific.

This patch removes the general addition of the flag to all runqemu
targets and adds the flag into the QB_OPT_APPEND for all the qemu*
machines in OE-Core that support its use (which is all of them).

(From OE-Core rev: e890c05e66a21702e9e8ccce794b74cb7f5518ed)

Signed-off-by: Nathan Rossi <nathan@nathanrossi.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>