qemu_process: Use unique directories for QMP processes

qemu_process: Use unique directories for QMP processes

Multiple QEMU processes for QMP commands can operate concurrently.

Use a unique directory under libDir for each QEMU process to avoid
pidfile and unix socket collision between processes.

The pid file name is changed from "capabilities.pidfile" to "qmp.pid"
because we no longer need to avoid a possible clash with a qemu domain
called "capabilities" now that the processes artifacts are stored in
their own unique temporary directories.

"Capabilities" was changed to "qmp" in the pid file name because these
processes are no longer specific to the capabilities usecase and are
more generic in terms of being used for any general purpose QMP message
exchanges with a QEMU process that is not associated with a domain.

Signed-off-by: Chris Venteicher <cventeic@redhat.com>
Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>