Detect maximum guests with AMD-SEV by libvirt API

AMD SEV-capable hardware has a fixed number of slots for holing encryption keys, thus supports limited number of guests with SEV enabled.
The [libvirt] num_memory_encrypted_guests option was initially added to make nova ware of that limit, because there was no libvirt API to obtain that limit[1], and if the option is not set then nova imposes no limit.

However the limit is presented by libvirt since libvirt 8.0.0[2][3] as part of domain capabilities.

```
# virsh domcapabilities
<domainCapabilities>
  ...
  <features>
    ...
    <sev supported='yes'>
      <cbitpos>47</cbitpos>
      <reducedPhysBits>1</reducedPhysBits>
      <maxGuests>100</maxGuests>
      <maxESGuests>15</maxESGuests>
    </sev>
    ...
  </features>
</domainCapabilities>
```

So now we can use this information to determine maximum number of SEV guests (as planned earlier) and reduce the options operators have to set properly.

Note that libvirt provides maximum number of SEV guests as well as one for SEV-ES guests, but because Nova supports only SEV guests now, we ignore the SEV-ES part.

[1] https://review.opendev.org/c/openstack/nova/+/666616
[2] https://gitlab.com/libvirt/libvirt/-/commit/34cb8f6fcd6a56a7bbcef2f7402def1682509e16
[3] https://gitlab.com/libvirt/libvirt/-/commit/7826148a72c97367fc6aaa76397fe92d32169723