KVM: x86/mmu: remove extended bits from mmu_role, rename field

mmu_role represents the role of the root of the page tables.
It does not need any extended bits, as those govern only KVM's
page table walking; the is_* functions used for page table
walking always use the CPU role.

ext.valid is not present anymore in the MMU role, but an
all-zero MMU role is impossible because the level field is
never zero in the MMU role.  So just zap the whole mmu_role
in order to force invalidation after CPUID is updated.

While making this change, which requires touching almost every
occurrence of "mmu_role", rename it to "root_role".

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