In set_cr4_guest_host_mask(), all cr4 pdptr bits are already set to be
intercepted in an unclear way.

Add X86_CR4_PDPTR_BITS to make it clear and self-documented.

No functionality changed.
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Message-Id: <20211108124407.12187-6-jiangshanlai@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>