x86/kvm: do not setup pv tlb flush when not paravirtualized

kvm_setup_pv_tlb_flush will waste memory and print a misguiding message
when KVM paravirtualization is not available.

Intel SDM says that the when cpuid is used with EAX higher than the
maximum supported value for basic of extended function, the data for the
highest supported basic function will be returned.

So, in some systems, kvm_arch_para_features will return bogus data,
causing kvm_setup_pv_tlb_flush to detect support for pv tlb flush.

Testing for kvm_para_available will work as it checks for the hypervisor
signature.

Besides, when the "nopv" command line parameter is used, it should not
continue as well, as kvm_guest_init will no be called in that case.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>