kvmtool: sample CPU endianness on virtio-mmio device reset

Save the CPU endianness when the device is reset. It is widely
assumed that the guest won't change its endianness after, or at
least not without reseting the device first.

A default implementation of the endianness sampling just returns
the default "host endianness" value so that unsuspecting architectures
are not affected.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>