KVM: PPC: Expand mmio_vsx_copy_type to cover VMX load/store element types

VSX MMIO emulation uses mmio_vsx_copy_type to represent VSX emulated
element size/type, such as KVMPPC_VSX_COPY_DWORD_LOAD, etc. This
patch expands mmio_vsx_copy_type to cover VMX copy type, such as
KVMPPC_VMX_COPY_BYTE(stvebx/lvebx), etc. As a result,
mmio_vsx_copy_type is also renamed to mmio_copy_type.

It is a preparation for reimplementing VMX MMIO emulation.

Signed-off-by: Simon Guo <wei.guo.simon@gmail.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>