KVM: x86: clear vcpu->run->hypercall.ret before exiting for KVM_EXIT_HYPERCALL

QEMU up to 9.2.0 is assuming that vcpu->run->hypercall.ret is 0 on exit and
it never modifies it when processing KVM_EXIT_HYPERCALL.  Make this explicit
in the code, to avoid breakage when KVM starts modifying that field.

This in principle is not a good idea... It would have been much better if
KVM had set the field to -KVM_ENOSYS from the beginning, so that a dumb
userspace that does nothing on KVM_EXIT_HYPERCALL would tell the guest it
does not support KVM_HC_MAP_GPA_RANGE.  However, breaking userspace is
a Very Bad Thing, as everybody should know.

Reported-by: Binbin Wu <binbin.wu@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>