x86: KVM guest: Add memory clobber to hypercalls

Hypercalls can modify arbitrary regions of memory.  Make sure to indicate this
in the clobber list.  This fixes a hang when using KVM_GUEST kernel built with
GCC 4.3.0.

This was originally spotted and analyzed by Marcelo.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>