KVM: selftests: Add helpers to allocate N pages of virtual memory

Add wrappers to allocate 1 and N pages of memory using de facto standard
values as the defaults for minimum virtual address, data memslot, and
page table memslot.  Convert all compatible users.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622200529.3650424-7-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>