gdbstub: Implement qemu physical memory mode

Add a new query/set which changes the memory GDB sees to physical memory
only.

gdb> maint packet qqemu.PhyMemMode
will reply the current phy_mem_mode state (1 for enabled, 0 for disabled)
gdb> maint packet Qqemu.PhyMemMode:1
Will make GDB read/write only to physical memory, set to 0 to disable

Signed-off-by: Jon Doron <arilou@gmail.com>
Message-Id: <20190529064148.19856-21-arilou@gmail.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>