KVM: x86/mmu: Derive shadow MMU page role from parent

Instead of computing the shadow page role from scratch for every new
page, derive most of the information from the parent shadow page.  This
eliminates the dependency on the vCPU root role to allocate shadow page
tables, and reduces the number of parameters to kvm_mmu_get_page().

Preemptively split out the role calculation to a separate function for
use in a following commit.

Note that when calculating the MMU root role, we can take
@role.passthrough, @role.direct, and @role.access directly from
@vcpu->arch.mmu->root_role. Only @role.level and @role.quadrant still
must be overridden for PAE page directories, when shadowing 32-bit
guest page tables with PAE page tables.

No functional change intended.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20220516232138.1783324-5-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>