crypto: Do not fail for EINTR during qcrypto_random_bytes

crypto: Do not fail for EINTR during qcrypto_random_bytes

We can always get EINTR for read; /dev/urandom is no exception.

Rearrange the order of tests for likelihood; allow degenerate buflen==0
case to perform a no-op zero-length read.  This means that the normal
success path is a straight line with a single test for success.

Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>