target-i386: check/enforce: Check all feature words

This adds the following feature words to the list of flags to be checked
by kvm_check_features_against_host():

 - cpuid_7_0_ebx_features
 - ext4_features
 - kvm_features
 - svm_features

This will ensure the "enforce" flag works as it should: it won't allow
QEMU to be started unless every flag that was requested by the user or
defined in the CPU model is supported by the host.

This patch may cause existing configurations where "enforce" wasn't
preventing QEMU from being started to abort QEMU. But that's exactly the
point of this patch: if a flag was not supported by the host and QEMU
wasn't aborting, it was a bug in the "enforce" code.

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