KVM: s390: Use ESCA instead of BSCA at VM init

All modern IBM Z and Linux One machines do offer support for the
Extended System Control Area (ESCA). The ESCA is available since the
z114/z196 released in 2010.
KVM needs to allocate and manage the SCA for guest VMs. Prior to this
change the SCA was setup as Basic SCA only supporting a maximum of 64
vCPUs when initializing the VM. With addition of the 65th vCPU the SCA
was needed to be converted to a ESCA.

Instead of allocating a BSCA and upgrading it for PV or when adding the
65th cpu we can always allocate the ESCA directly upon VM creation
simplifying the code in multiple places as well as completely removing
the need to convert an existing SCA.

In cases where the ESCA is not supported (z10 and earlier) the use of
the SCA entries and with that SIGP interpretation are disabled for VMs.
This increases the number of exits from the VM in multiprocessor
scenarios and thus decreases performance.
The same is true for VSIE where SIGP is currently disabled and thus no
SCA entries are used.

The only downside of the change is that we will always allocate 4 pages
for a 248 cpu ESCA instead of a single page for the BSCA per VM.
In return we can delete a bunch of checks and special handling depending
on the SCA type as well as the whole BSCA to ESCA conversion.

With that behavior change we are no longer referencing a bsca_block in
kvm->arch.sca. This will always be esca_block instead.
By specifying the type of the sca as esca_block we can simplify access
to the sca and get rid of some helpers while making the code clearer.

KVM_MAX_VCPUS is also moved to kvm_host_types to allow using this in
future type definitions.

Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Christoph Schlameuss <schlameuss@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>