oeqa/qemurunner: Handle path length issues for qmp socket

After the addition of the qmp socket, runqemu started failing:

ERROR - Failed to run qemu: qemu-system-aarch64: -qmp unix:/home/yocto/actions-runner-meta-openembedded/_work/meta-openembedded/meta-openembedded/yoe/build/tmp/.3eg5fiid,server,wait:
UNIX socket path '/home/yocto/actions-runner-meta-openembedded/_work/meta-openembedded/meta-openembedded/yoe/build/tmp/.3eg5fiid' is too long
Path must be less than 108 bytes

To avoid this, run qemu within tmpdir and use a relative path to the socket.
This avoids having to patch the socket code within qemu.

Update the client code to chdir and only use a relative path to the socket
to match.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>