Description of problem:
It was said that when using SEV, there is a limit of 15 guests that are able to use the feature concurrently.

https://www.redhat.com/archives/libvir-list/2019-January/msg00652.html

This kind of limit is important for a mgmt application to know about, so that it can place VMs on hosts which have suitable resource available, but this is not yet exposed anywhere.

It needs to be reported in the libvirt capabilities XML. Probably we would get this information from QEMU via QMP when querying SEV support.

Version-Release number of selected component (if applicable):
libvirt-5.0.0-6.el8

How reproducible:
Always

Steps to Reproduce:
1. Run  'virsh domcapabilities kvm'

Actual results:
SEV feature is reported, but with no mention of the guest limit:

     <domainCapabilities>
          ...
          <features>
            ...
            <sev supported='yes'/>
              ...
            </sev>
          </features>
        </domainCapabilities>

Expected results:
Above XML includes the guest limit