I had similar problem in the past with ESXi 6.0 U2 host where I defined a VM and used it as oVirt node for nested virtualization. If I run on this "virtual" node an L2 VM all went well until custom emulated machine value was at "pc-i440fx-rhel7.2.0".

At some point the default became 7.3 and the L2 VM was not able to boot any more: it remained in blank screen at boot.

The same up to now in 4.2.6.1 where this default seems to be 7.5 

I'm having same problem in a mini lab on a nuc6 where I have ESXi 6.7 installed and try to setup an hosted engine environment.

This problem prevents me to configure the hosted engine because the L2 VM that should be the hosted engine freezes (?) during its first startup and I see indeed that the qemu-kvm process is started with option "-machine pc-i440fx-rhel7.5.0"

Being stated that I'm in an unsupported configuration, I would like to understand if I can correct in some point and proceed.

So some questions:

1) How can I understand what is the difference between running qemu-kvm with  "-machine pc-i440fx-rhel7.2.0" ad  "-machine pc-i440fx-rhel7.5.0" so that I can then dig with ESXi parameters eventually to tune the settings of its VM that becomes my oVirt hypervisor?

2) Do I have any chance to configure my oVirt node so that when it runs the hosted-engine deploy it runs the engine VM with 7.2 parameter? Any conf file on host where I can somehow force 7.2 compatibility or set it as default?

3) during gluster base hosted-engine deployment at first "local" startup how can I connect to the console of the hosted engine VM and see where it is blocked?

Thanks in advance,

Gianluca