KVM: selftests: Introduce VM_MODE_PXXV48_4K

The naming VM_MODE_P52V48_4K is explicit but unclear when used on
x86_64 machines, because x86_64 machines are having various physical
address width rather than some static values.  Here's some examples:

  - Intel Xeon E3-1220:  36 bits
  - Intel Core i7-8650:  39 bits
  - AMD   EPYC 7251:     48 bits

All of them are using 48 bits linear address width but with totally
different physical address width (and most of the old machines should
be less than 52 bits).

Let's create a new guest mode called VM_MODE_PXXV48_4K for current
x86_64 tests and make it as the default to replace the old naming of
VM_MODE_P52V48_4K because it shows more clearly that the PA width is
not really a constant.  Meanwhile we also stop assuming all the x86
machines are having 52 bits PA width but instead we fetch the real
vm->pa_bits from CPUID 0x80000008 during runtime.

We currently make this exclusively used by x86_64 but no other arch.

As a slight touch up, moving DEBUG macro from dirty_log_test.c to
kvm_util.h so lib can use it too.

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