The -virtfs option creates an fsdev representing the pass-through file
system and a guest-visible virtio-9p-pci device that can access this
file system.  This patch replaces the string manipulation used to build
and reparse option lists with direct QemuOpts calls.  Removing the
string manipulation code makes it easier to maintain and less error
prone.

An error message is also updated to use "mount_tag" instead of
"mnt_tag".
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NVenkateswararao Jujjuri <jvrao@linux.vnet.ibm.com>