kvm: drop unsupported capabilities, fix documentation

No kernel ever reported KVM_CAP_DEVICE_MSIX, KVM_CAP_DEVICE_MSI,
KVM_CAP_DEVICE_ASSIGNMENT, KVM_CAP_DEVICE_DEASSIGNMENT.

This makes the documentation wrong, and no application ever
written to use these capabilities has a chance to work correctly.
The only way to detect support is to try, and test errno for ENOTTY.
That's unfortunate, but we can't fix the past.

Document the actual semantics, and drop the definitions from
the exported header to make it easier for application
developers to note and fix the bug.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>