xen-tools -- libxl leak of pv kernel and initrd on error

The Xen Project reports:

When constructing a guest which is configured to use a PV
bootloader which runs as a userspace process in the toolstack domain
(e.g. pygrub) libxl creates a mapping of the files to be used as
kernel and initial ramdisk when building the guest domain.
However if building the domain subsequently fails these mappings
would not be released leading to a leak of virtual address space in
the calling process, as well as preventing the recovery of the
temporary disk files containing the kernel and initial ramdisk.
For toolstacks which manage multiple domains within the same
process, an attacker who is able to repeatedly start a suitable
domain (or many such domains) can cause an out-of-memory condition in the
toolstack process, leading to a denial of service.
Under the same circumstances an attacker can also cause files to
accumulate on the toolstack domain filesystem (usually under /var in
dom0) used to temporarily store the kernel and initial ramdisk,
perhaps leading to a denial of service against arbitrary other
services using that filesystem.