libqtest: Rename functions to send QMP messages

libqtest: Rename functions to send QMP messages

The functions to receive messages are called qtest_qmp_receive() and
qmp_receive(), qmp_fd_receive().  The ones to send messages are called
qtest_async_qmp(), qtest_async_qmpv(), qmp_async(), qmp_fd_send(),
qmp_fd_sendv().  Inconsistent.  Rename the *_async* ones to
qmp_send(), qtest_qmp_send(), qtest_qmp_vsend().  Rename
qmp_fd_sendv() to qmp_fd_vsend().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180806065344.7103-2-armbru@redhat.com>