Handle all memslot0 size adjustments in __vm_create().  Currently, the
adjustments reside in __vm_create_with_vcpus(), which means tests that
call vm_create() or __vm_create() directly are left to their own devices.
Some tests just pass DEFAULT_GUEST_PHY_PAGES and don't bother with any
adjustments, while others mimic the per-vCPU calculations.

For vm_create(), and thus __vm_create(), take the number of vCPUs that
will be runnable to calculate that number of per-vCPU pages needed for
memslot0.  To give readers a hint that neither vm_create() nor
__vm_create() create vCPUs, name the parameter @nr_runnable_vcpus instead
of @nr_vcpus.  That also gives readers a hint as to why tests that create
larger numbers of vCPUs but never actually run those vCPUs can skip
straight to the vm_create_barebones() variant.
Signed-off-by: NSean Christopherson <seanjc@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>