runqemu-internal: correctly set format for root FS as raw

qemu guesses via probing the format of root FS, but
gives a warning and restricts write operations on block 0.
Fix it by setting correctly the format as raw for more
machines and non-KVM machines.
In some cases, replaced the way machine disk is set for qemu.

Fix for [YOCTO #7918]

Signed-off-by: Cristian Iorga <cristian.iorga@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>