Implements functions for both HMP and QMP mode.

For HMP mode, qemu uses "M" as the units by default, so the passed "sized"
is divided by 1024.

For QMP mode, qemu uses "Bytes" as the units by default, the passed "sized"
is multiplied by 1024.

All of the monitor functions return -1 on failure, 0 on success, or -2 if
not supported.