user: move common-user includes to a subdirectory of {bsd,linux}-user/

Avoid polluting the compilation of common-user/ with local include files;
making an include file available to common-user/ should be a deliberate
decision in order to keep a clear interface that can be used by both
bsd-user/ and linux-user/.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>