passthrough: Fix PCI hot-plug option parsing

passthrough: Fix PCI hot-plug option parsing

When a PCI function is passed-through extra options may be passed
through.
In the case of boot-time PCI pass-through the documented format is:
    [dom:]bus:dev.slot[@vslot][[,opt]...]
    e.g.
    00:01.00.1@7,msitranslate=3D1

In the case of PCI hot-plug the xm pci-attach command take the
following arguments:
    [-o opt[,opt]...] [dom:]bus:dev.slot [vslot]
    e.g.
    -o msitranslate=3D1 00:01.00.1 7

    These xm ends up passing these to xem-qemu as:
    [dom:]bus:dev.slot[[,opt]...][@vslot]
    e.g.
    00:01.00.1,msitranslate=3D1@7

    Note that the option and the vslot have are transposed when
    compared to the format used by boot-time PCI pass-through.

The parser inside qemu-xen can only handle the format used by
boot-time PCI pass-through and because of this ignores
any options passed by hot-plug.

This patch alters format used by hot-plug to match the parser.

Signed-off-by: Simon Horman <horms@verge.net.au>