kvm: selftests: ucall: fix exit mmio address guessing

Fix two more bugs in the exit_mmio address guessing.
The first bug was that the start and step calculations were
wrong since they were dividing the number of address bits instead
of the address space. The second other bug was that the guessing
algorithm wasn't considering the valid physical and virtual address
ranges correctly for an identity map.

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