KVM: Fix guest register corruption on paravirt hypercall

The hypercall code mixes up the ->cache_regs() and ->decache_regs()
callbacks, resulting in guest register corruption.

Signed-off-by: Dor Laor <dor.laor@qumranet.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>