Add map client retry notification (Avi Kivity)

The target memory mapping API may fail if the bounce buffer resources
are exhausted.  Add a notification mechanism to allow clients to retry
the mapping operation when resources become available again.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6395 c046a42c-6fe2-441c-8c8c-71466251a162
