docs: formatdomain: explain host-model/host-passthrough requirements
docs: formatdomain: explain host-model/host-passthrough requirements

host-passthrough documentation menions that the source and destination
hosts are not identical in both hardware and configuration.  Configuration
actually includes microcode version and QEMU version, but this is not
clear so make it explicit

Signed-off-by: Paolo Bonzini <69d3ebcf5ee4e7f05a50dc23cd96655b730804b4@redhat.com>
Message-Id: <20190802125415.15227-1-69d3ebcf5ee4e7f05a50dc23cd96655b730804b4@redhat.com>
Reviewed-by: Jiri Denemark <62bdf77dc47919a4d59a91822129d14633cfca81@redhat.com>

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