Add a troubleshooting section to the Xen chapter.

In the discussion leading up to r52813, rgrimes@ suggested to add a
troubleshooting section to the Xen chapter to help with debugging issues.

In particular, users should know where Xen 4.11 and higher logs important
information relevant for debugging. There are also a couple of options
that increase the verbosity of the dom0 and enable iommu debugging. These
were divided up into two subsections: one for the host and the other for
the guest.

Thanks go out to royger@ for providing the information and putting together
a documentation patch before I could.

Suggested by: rgrimes
Patch by: royger
Approved by: bcr
Differential Revision: https://reviews.freebsd.org/D19107