The default NIC model for QEMU varies per machine type, and is liable to
change across machine type versions. Documenting e1000 NIC as the
default for PC/i386 is thus misleading to users at best. In particular
the PC q35 machine type switched to use e1000e, but only in machine
type versions after 2.11.

Rather than try to explain which NIC model is used for each machine
type version, remove mention of e1000 as the default, and steer users
towards always specifying their desired model.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed by: Aleksandar Markovic <amarkovic@wavecomp.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>