alpha-softmmu: Disable for the 0.15 release branch.

The system emulation code was not merged before the branch.
Let's leave that work for the next release.
Signed-off-by: NRichard Henderson <rth@twiddle.net>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

configure

@@ -846,7 +846,6 @@ if [ "$softmmu" = "yes" ] ; then
     default_target_list="\
 i386-softmmu \
 x86_64-softmmu \
-alpha-softmmu \
 arm-softmmu \
 cris-softmmu \
 lm32-softmmu \