selftests: kvm: Mmap the entire vcpu mmap area

The vcpu mmap area may consist of more than just the kvm_run struct.
Allocate enough space for the entire vcpu mmap area. Without this, on
x86, the PIO page, for example, will be missing.  This is problematic
when dealing with an unhandled exception from the guest as the exception
vector will be incorrectly reported as 0x0.

Message-Id: <20210210165035.3712489-1-aaronlewis@google.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Co-developed-by: Steve Rutherford <srutherford@google.com>
Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>