KVM: x86: merge handle_mmio_page_fault and handle_mmio_page_fault_common

They are exactly the same, except that handle_mmio_page_fault
has an unused argument and a call to WARN_ON.  Remove the unused
argument from the callers, and move the warning to (the former)
handle_mmio_page_fault_common.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>