target/i386: introduce generic feature dependency mechanism

Sometimes a CPU feature does not make sense unless another is
present.  In the case of VMX features, KVM does not even allow
setting the VMX controls to some invalid combinations.

Therefore, this patch adds a generic mechanism that looks for bits
that the user explicitly cleared, and uses them to remove other bits
from the expanded CPU definition.  If these dependent bits were also
explicitly *set* by the user, this will be a warning for "-cpu check"
and an error for "-cpu enforce".  If not, then the dependent bits are
cleared silently, for convenience.

With VMX features, this will be used so that for example
"-cpu host,-rdrand" will also hide support for RDRAND exiting.

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

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