exec: check kvm mmu notifiers earlier

Move kvm mmu notifiers check before calling file_ram_alloc(), with the
other xen precondition. (file_ram_alloc() will be reused in other cases
than -mem-path).

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20170602141229.15326-2-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
