hvf: Add missing break statement

In target/i386/hvf/hvf.c, a break statement was probably missing in
`hvf_vcpu_exec()`, in handling EXIT_REASON_HLT.

These lines seemed to be equivalent to `kvm_handle_halt()`.

Signed-off-by: Chen Zhang <tgfbeta@me.com>
Message-Id: <087F1D9C-109D-41D1-BE2C-CE5D840C981B@me.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/i386/hvf/hvf.c

@@ -708,6 +708,7 @@ int hvf_vcpu_exec(CPUState *cpu)
                 !(idtvec_info & VMCS_IDT_VEC_VALID)) {
                 cpu->halted = 1;
                 ret = EXCP_HLT;
+                break;
             }
             ret = EXCP_INTERRUPT;
             break;