gdbstub: Implement generic set/query (Q/q pkt) with new infra

gdbstub: Implement generic set/query (Q/q pkt) with new infra

The generic set/query packets contains implementation for varioius
sub-commands which are required for GDB and also additional commands
which are QEMU specific.

To see which QEMU specific commands are available use the command
gdb> maintenance packet qqemu.Supported

Currently the only implemented QEMU specific command is the command
that sets the single step behavior.

gdb> maintenance packet qqemu.sstepbits
Will display the MASK bits used to control the single stepping.

gdb> maintenance packet qqemu.sstep
Will display the current value of the mask used when single stepping.

gdb> maintenance packet Qqemu.sstep:HEX_VALUE
Will change the single step mask.

Signed-off-by: Jon Doron <arilou@gmail.com>
Message-Id: <20190529064148.19856-18-arilou@gmail.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>