qemuBuildDriveDevStr: Prefer default aliases for IDE bus

qemuBuildDriveDevStr: Prefer default aliases for IDE bus

https://bugzilla.redhat.com/show_bug.cgi?id=1434451

When testing user aliases it was discovered that for 440fx
machine type which has default IDE bus builtin, domain cannot
start if IDE controller has the user provided alias. This is
because for 440fx we don't put the IDE controller onto the
command line (since it is builtin) and therefore any device that
is plugged onto the bus must use the default alias.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>