target-i386: kvm: Use env->mcg_cap when setting up MCE

When setting up MCE, instead of using the MCE_*_DEF macros
directly, just filter the existing env->mcg_cap value.

As env->mcg_cap is already initialized as
MCE_CAP_DEF|MCE_BANKS_DEF at target-i386/cpu.c:mce_init(), this
doesn't change any behavior. But it will allow us to change
mce_init() in the future, to implement different defaults
depending on CPU model, machine-type or command-line parameters.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1448471956-66873-9-git-send-email-pbonzini@redhat.com>
