KVM: x86/mmu: Allocate pae_root and lm_root pages in dedicated helper

Move the on-demand allocation of the pae_root and lm_root pages, used by
nested NPT for 32-bit L1s, into a separate helper.  This will allow a
future patch to hold mmu_lock while allocating the non-special roots so
that make_mmu_pages_available() can be checked once at the start of root
allocation, and thus avoid having to deal with failure in the middle of
root allocation.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210305011101.3597423-6-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>