fix variable type in qemu-io.c

The variable len can get a negative return value from cvtnum,
which we check for, but which is impossible with the current
unsigned variable type.  Currently the if(len < 0) check is
pointless.  This patch fixes that.

Signed-off-by: Joel Schopp <jschopp@austin.ibm.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
