Configurable Machine for MIPS, i386, X86_64

[AndrewFasano]

Port of configurable-machine features from PANDA, updated to hopefully work with QEMU 5.1.

Note that although this PR contains a configurable mcahine for the i386 and x86_64-softmmu targets, those don't currently build in avatar-qemu. It's probably a simple missing #ifdef target_xxx guard, but I'm not too familiar with the codebase:

  LINK    i386-softmmu/qemu-system-i386
hw/avatar/interrupts.o: In function `avatar_armv7m_nvic_forward_write':
/home/fasano/git/avatar-qemu/hw/avatar/interrupts.c:58: undefined reference to `get_current_pc'
/home/fasano/git/avatar-qemu/hw/avatar/interrupts.c:63: undefined reference to `rmem_tx_queue_ref'
/home/fasano/git/avatar-qemu/hw/avatar/interrupts.c:63: undefined reference to `qemu_avatar_mq_send'
/home/fasano/git/avatar-qemu/hw/avatar/interrupts.c:64: undefined reference to `rmem_rx_queue_ref'
/home/fasano/git/avatar-qemu/hw/avatar/interrupts.c:64: undefined reference to `qemu_avatar_mq_receive'

Someone who knows how to use avatar-qemu should probably test the MIPS configurable machine before merging, I only made sure it compiled. Looks like a bunch of the QEMU APIs related to CPU creation changed (here?).