Currently we deny any VHOST_* functionality if the architecture
supports guests with different endianness than the host. Most of the
time even on those architectures the endianness of guest and host are
the same, though, so we are denying the glory of VHOST needlessly.
Switch from compile time determination to a run time scheme, which
takes the actual endianness of the guest into account.
For this we change the semantics of VIRTIO_ENDIAN_HOST to return the
actual endianness of the host (the endianness of kvmtool at compile
time, really). The actual check in vhost_net now compares this against
the guest endianness.
This enables vhost support on ARM and ARM64.

Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>