qemu: Fix off-by-one error while unescaping monitor strings

qemu: Fix off-by-one error while unescaping monitor strings

While unescaping the commands the commands passed through to the monitor
function qemuMonitorUnescapeArg() initialized lenght of the input string
to strlen()+1 which is fine for alloc but not for iteration of the
string.

This patch fixes the off-by-one error and drops the pointless check for
a single trailing slash that is automaticaly handled by the default
branch of switch.
(cherry picked from commit 0f4660c8787cc41fe67f869984c0ae11d680037e)