kvm: check KVM_CAP_SYNC_MMU with kvm_vm_check_extension()

On a server-class ppc host, this capability depends on the KVM type,
ie, HV or PR. If both KVM are present in the kernel, we will always
get the HV specific value, even if we explicitely requested PR on
the command line.

This can have an impact if we're using hugepages or a balloon device.

Since we've already created the VM at the time any user calls
kvm_has_sync_mmu(), switching to kvm_vm_check_extension() is
enough to fix any potential issue.

It is okay for the other archs that also implement KVM_CAP_SYNC_MMU,
ie, mips, s390, x86 and arm, because they don't depend on the VM being
created or not.

While here, let's cache the state of this extension in a bool variable,
since it has several users in the code, as suggested by Thomas Huth.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <150600965332.30533.14702405809647835716.stgit@bahia.lan>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
