igvm_defs: indicate that it is legal for a pagetable region to lie within a relocation region

[chris-oo]

The spec today does not allow this, but some of our loader implementations allow this. The reason being:

1. It's important to allow marking a larger region as relocatable, as some headers may describe the larger region as relocatable, say for required memory.
2. We want to describe that pagetables should be relocated with another region, and relocated the same as that outer region.

The update here is to change the description for IGVM_VHS_PAGE_TABLE_RELOCATION to allow it to lie within another relocation region, and indicate that it must be relocated with the outer region it's included in.

[chris-oo]

I would like to think about this a bit though before we commit to this.