target-ppc: extract register length calculation in gdbstub

This patch extracts the method to determine a register's size
into a separate function.

Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Thomas Falcon <tlfalcon@linux.vnet.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
target-ppc: gdbstub allow byte swapping for reading/writing registers

This patch allows registers to be properly read from and written to
when using the gdbstub to debug a ppc guest running in little
endian mode.

Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Thomas Falcon <tlfalcon@linux.vnet.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>