target-i386: do not memcpy in and out of xmm_regs

After the next patch, we will move the high parts of AVX and AVX512 registers
in the same array as the SSE registers.  This will make it impossible to
memcpy an array of 128-bit values in and out of xmm_regs in one swoop.
Use a for loop instead.

Similarly, always use XMM_Q in translate.c.  This avoids introducing bugs
such as the one fixed in the previous patch.

Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>