qemu: net/drive add/remove tweaks (Marcelo Tosatti)

Export net/drive add/remove functions for device hotplug usage.

Return the table index on add.

Return failure instead of exiting if limit has been reached
on drive_add.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6599 c046a42c-6fe2-441c-8c8c-71466251a162