target-i386: Disable kvm_mmu by default

target-i386: Disable kvm_mmu by default

KVM_CAP_PV_MMU capability reporting was removed from the kernel since
v2.6.33 (see commit a68a6a7282373), and was completely removed from the
kernel since v3.3 (see commit fb92045843). It doesn't make sense to keep
it enabled by default, as it would cause unnecessary hassle when using
the "enforce" flag.

This disables kvm_mmu on all machine-types. With this fix, the possible
scenarios when migrating from QEMU <= 1.3 to QEMU 1.4 are:

------------+----------+----------------------------------------------------
 src kernel | dst kern.| Result
------------+----------+----------------------------------------------------
 >= 2.6.33  | any      | kvm_mmu was already disabled and will stay disabled
 <= 2.6.32  | >= 3.3   | correct live migration is impossible
 <= 2.6.32  | <= 3.2   | kvm_mmu will be disabled on next guest reboot *
------------+----------+----------------------------------------------------

 * If they are running kernel <= 2.6.32 and want kvm_mmu to be kept
   enabled on guest reboot, they can explicitly add +kvm_mmu to the QEMU
   command-line. Using 2.6.33 and higher, it is not possible to enable
   kvm_mmu explicitly anymore.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>