KVM: x86: move definition PT_MAX_HUGEPAGE_LEVEL and KVM_NR_PAGE_SIZES together

Currently, there are two definitions related to huge page, but a little bit
far from each other and seems loosely connected:

 * KVM_NR_PAGE_SIZES defines the number of different size a page could map
 * PT_MAX_HUGEPAGE_LEVEL means the maximum level of huge page

The number of different size a page could map equals the maximum level
of huge page, which is implied by current definition.

While current implementation may not be kind to readers and further
developers:

 * KVM_NR_PAGE_SIZES looks like a stand alone definition at first sight
 * in case we need to support more level, two places need to change

This patch tries to make these two definition more close, so that reader
and developer would feel more comfortable to manipulate.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>