KVM: Fix emulate_sys[call, enter, exit]()'s fault handling

This patch fixes emulate_syscall(), emulate_sysenter() and
emulate_sysexit() to handle injected faults properly.

Even though original code injects faults in these functions,
we cannot handle these unless we use the different return
value from the UNHANDLEABLE case. So this patch use X86EMUL_*
codes instead of -1 and 0 and makes x86_emulate_insn() to
handle these propagated faults.

Be sure that, in x86_emulate_insn(), goto cannot_emulate and
goto done with rc equals X86EMUL_UNHANDLEABLE have same effect.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>