q35: change default NIC to e1000e

The e1000 NIC is getting old and is not a very good default for a
PCIe machine type.  Change it to e1000e, which should be supported
by a good number of guests.

In particular, drivers for 82574 were added first to Linux 2.6.27 (2008)
and Windows 2008 R2.  This does mean that Windows 2008 will not work
anymore with Q35 machine types and a default "-net nic -net xxx" network
configuration; it did work before because it does have an AHCI driver.
However, Windows 2008 has been declared out of main stream support
in 2015.  It will get out of extended support in 2020.  Windows 2008
R2 has the same end of support dates and, since the two are basically
Vista vs. Windows 7, R2 probably is more popular.

Reviewed-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>