Fix BAR2-BAR3

[AndrewGavril]

Reconfiguration of BAR 2 leads to errors in the operation of the device.

[mxkrsv]

I re-checked, and actually disabling ivshmem_bar2 registering in guest PCI space doesn't break anything.

That potentially allows us to register BARs on regions 2 and 3.

There's a new problem, however. When registering 4 BARs, guest system boot hangs. I haven't yet looked at kernel logs.

However, 3 BARs is still fine, and also their indices don't matter.

So, for now, we can register a total of 3 BARs 256MiB each.

[mxkrsv]

Pushed the third bar enablement changes to #78