target-i386: fix disassembly with PAE=1, PG=0

CR4.PAE=1 will not enable paging if CR0.PG=0, but the "if" chain
in x86_cpu_get_phys_page_debug says otherwise.  Check CR0.PG
before everything else.

Fixes "-d in_asm" for a code section at the beginning of OVMF.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Max Filippov <jcmvbkbc@gmail.com>
