qemu: Move creation and opening of chardev backend FDs to host prepare step

qemu: Move creation and opening of chardev backend FDs to host prepare step

The opening of files for FD passing for a chardev backend was
historically done in the function which is formatting the commandline.

This has multiple problems. Firstly the function takes a lot of
parameters which need to be passed through the commandline formatters.
This made the 'qemuBuildChrChardevStr' extremely unappealing to the
extent that we have multiple other custom formatters in places which
didn't really want to use the function.

Additionally the function is also creating files in the host in certain
configurations which is wrong for a commandline formatter to do. This
meant that e.g. not all chardev test cases can be converted to use
DO_TEST_CAPS_LATEST as we attempt to use such code path and attempt to
create files outside of the test directory.

This patch moves the opening of the filedescriptors from
'qemuBuildChrChardevFileStr' into a new helper
'qemuProcessPrepareHostBackendChardevOne' which is called using
'qemuDomainDeviceBackendChardevForeach'.

To preserve test behaviour we also have another instance
'testPrepareHostBackendChardevOne' which is populating mock
filedescriptors.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>