refactor: VirtIO MMIO persistence logic

VirtIO MMIO restore logic activates the device the moment we restore the
device state, if the device was activated when snapshotted. Move the
activation responsibility to the logic the restores the MMIO transport.
The reason for this change is that that's how it will be done for the
PCI transport. Unifying this will allow us reusing the same types for
restoring the non-transport state of devices.

Also, simplify a bit the types that we use for serializing/deserializing
the state of a device.

Signed-off-by: Babis Chalios <[email protected]>

Author: bchalios