x86: Let tests omit PT_USER_MASK when configuring virtual memory

Let tests opt out of setting PT_USER_MASK so that they can set CR4.SMEP
and/or CR4.SMAP without having to manually modify all PTEs, which is
beyond painful.  Keep user pages the default to avoid having to update
existing tests.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622210047.3691840-11-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>