Fix tap downscript argument (Mark McLoughlin)

Fix tap downscript argument (Mark McLoughlin)

Kill off the hack that parses info_str for the tap interface
name to pass as the argument to the downscript and, instead,
just explicitly keep a copy of the string for later.

As reported by John Wong, this commit:

  Add qemu_format_nic_info_str()

changed the invocation of downscript from e.g.

  /path/kvm-ifdown "tap0"

to:

  /path/kvm-ifdown "tap0,script=/path/kvm-ifup,downscript=/path/kvm-ifdown"

This fix restores the original behavior.

Reported-by: John Wong <johnw@wonghome.net>
Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6285 c046a42c-6fe2-441c-8c8c-71466251a162