x86/PVH: replace bogus assertion with conditional

While PVH guests currently have to start in 64-bit mode, nothing keeps
them from entering compatibility mode via a suitable ring-0 code
selector and making a hypercall from there. Fail such attempts rather
than asserting they won't happen.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -4876,10 +4876,10 @@ int hvm_do_hypercall(struct cpu_user_reg
                                                    regs->r8, regs->r9);
         curr->arch.hvm_vcpu.hcall_64bit = 0;
     }
+    else if ( unlikely(is_pvh_vcpu(curr)) )
+        regs->_eax = -ENOSYS; /* PVH 32bitfixme. */
     else
     {
-        ASSERT(!is_pvh_vcpu(curr));   /* PVH 32bitfixme. */
-
         HVM_DBG_LOG(DBG_LEVEL_HCALL, "hcall%u(%x, %x, %x, %x, %x, %x)", eax,
                     (uint32_t)regs->ebx, (uint32_t)regs->ecx,
                     (uint32_t)regs->edx, (uint32_t)regs->esi,