qemuBuildChannelsCommandLine: Extract common formatting of 'chardev'
qemuBuildChannelsCommandLine: Extract common formatting of 'chardev'

Both active branches create the same backend chardev. Since there is no
other case, extract it before the switch so that we don't have to
duplicate it.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Eric Blake <a9da18479c871fe903d9590822cfe2629c6a4d11@redhat.com>

crobinso/libvirt,libvirt/libvirt,zippy2/libvirt,crobinso/libvirt,jfehlig/libvirt,jfehlig/libvirt,crobinso/libvirt,nertpinx/libvirt,nertpinx/libvirt,zippy2/libvirt,olafhering/libvirt,nertpinx/libvirt,olafhering/libvirt,jardasgit/libvirt,jfehlig/libvirt,olafhering/libvirt,zippy2/libvirt,libvirt/libvirt,libvirt/libvirt,zippy2/libvirt,jardasgit/libvirt,nertpinx/libvirt,libvirt/libvirt,jardasgit/libvirt,jardasgit/libvirt,crobinso/libvirt,olafhering/libvirt,jfehlig/libvirt,nertpinx/libvirt,jardasgit/libvirt