KVM: selftests: Lower the min virtual address for misc page allocations

Reduce the minimum virtual address of page allocations from 0x10000 to
KVM_UTIL_MIN_VADDR (0x2000).  Both values appear to be completely
arbitrary, and reducing the min to KVM_UTIL_MIN_VADDR will allow for
additional consolidation of code.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622200529.3650424-8-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>