megasas: disable due to build breakage

The Buildbot has detected a new failure on builder default_i386_rhel61 while
building qemu.

Full details are available at:
 http://buildbot.b1-systems.de/qemu/builders/default_i386_rhel61/builds/304



The proper fix is non-trivial so let's disable the build by default until it's
fixed properly.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>