virtiofs support for kexec-tools
    
    This patch add virtiofs support for kexec-tools by introducing a new option
    for /etc/kdump.conf:
    
    virtiofs myfs
    
    Where myfs is a variable tag name specified in qemu cmdline
    "-device vhost-user-fs-pci,tag=myfs".
    
    The patch covers the following cases:
    1) Dumping VM's vmcore to a virtiofs shared directory;
    2) When the VM's rootfs is a virtiofs shared directory and dumping the
       VM's vmcore to its subdirectory, such as /var/crash;
    3) The combination of case 1 & 2: The VM's rootfs is a virtiofs shared
       directory and dumping the VM's vmcore to another virtiofs shared
       directory.
    
    Case 2 & 3 need dracut >= 057, otherwise VM cannot boot from virtiofs
    shared rootfs. But it is not the issue of kexec-tools.
    
    Reviewed-by: Philipp Rudo <prudo@redhat.com>
    Signed-off-by: Tao Liu <ltao@redhat.com>