Let's be consistent and call it hmp-commands.hx, so that we have
qmp-commands.hx for QMP and hmp-commands.hx for HMP.

Please, note that this commit doesn't touch qemu-monitor.texi. All
texi files have the qemu- prefix and I don't think it's worth
changing that.
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>