qemu: pass only host arch instead of the whole virCaps

qemu: pass only host arch instead of the whole virCaps

This is a preparation for following patches where we switch to
virFileCache for QEMU capabilities cache

The host arch will always remain the same but virCaps may change.  Now
the host arch is stored while creating new qemu capabilities cache.
It removes the need to pass virCaps into virQEMUCapsCache*() functions.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>