kvm: Fix enable_cap helpers on older gcc

Commit 40f1ee27aa1 introduced handy helpers for enable_cap calls on
vcpu and vm level. Unfortunately some older gcc versions (4.7.1, 4.6)
seem to choke on signedness detection in inline created variables:

target-ppc/kvm.c: In function 'kvmppc_booke_watchdog_enable':
target-ppc/kvm.c:1302:21: error: comparison of unsigned expression < 0 is always false [-Werror=type-limits]
target-ppc/kvm.c: In function 'kvmppc_set_papr':
target-ppc/kvm.c:1504:21: error: comparison of unsigned expression < 0 is always false [-Werror=type-limits]

However - thanks to Thomas Huth for the suggestion - we can just cast the
offending potentially 0 value to a signed type, making the comparison signed.

Reviewed-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
