kvm: selftests: actually use all of lib/vmx.c

The allocation of the VMXON and VMCS is currently done twice, in
lib/vmx.c and in vmx_tsc_adjust_test.c.  Reorganize the code to
provide a cleaner and easier to use API to the tests.  lib/vmx.c
now does the complete setup of the VMX data structures, but does not
create the VM or set CPUID.  This has to be done by the caller.

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