sendkey: use consistent API convention

sendkey: use consistent API convention

Even though rpc uses 'unsigned int' for the _len parameter that
passes the length of item<length>, the public libvirt APIs all
use 'int' and filter out lengths < 0, except for virDomainSendKey.

* include/libvirt/libvirt.h.in (virDomainSendKey): All other APIs
use int for array length.
* src/libvirt.c (virDomainSendKey): Adjust.
* src/driver.h (virDrvDomainSendKey): Likewise.
* daemon/remote_generator.pl: Likewise.