cmd/qemu: Enable additional accelerators
cmd/qemu: Enable additional accelerators

Upstream qemu supports accelerators other than kvm. Allow the user
to choose. On Linux we still default to 'lvm' if available. On
macOS we try the new 'hvf' accelerator, if available.

Disable acceleration if the host arch does not match requested
qemu arch.

Also change the LINUXKIT_QEMU_KVM env var to LINUXKIT_QEMU_ACCEL
and use the functions in utils.go for env var overrides.

Signed-off-by: Rolf Neugebauer <6fdd1d8677ab4fbcb4df3eff6e190c3298f7f742@docker.com>

JohnnyLeone/linuxkit,konstruktoid/linuxkit,furious-luke/linuxkit,YuPengZTE/linuxkit,justincormack/linuxkit,ndauten/linuxkit,rn/linuxkit,furious-luke/linuxkit,YuPengZTE/linuxkit,thebsdbox/linuxkit,justincormack/linuxkit,djs55/linuxkit,furious-luke/linuxkit,YuPengZTE/linuxkit,djs55/linuxkit,thebsdbox/linuxkit,linuxkit/linuxkit,djs55/linuxkit,ndauten/linuxkit,rn/linuxkit,ndauten/linuxkit,rn/linuxkit,zx2c4/linuxkit,linuxkit/linuxkit,JohnnyLeone/linuxkit,konstruktoid/linuxkit,konstruktoid/linuxkit,furious-luke/linuxkit,yankcrime/linuxkit,rn/linuxkit,deitch/linuxkit,justincormack/linuxkit,zx2c4/linuxkit,rn/linuxkit,zx2c4/linuxkit,konstruktoid/linuxkit,deitch/linuxkit,linuxkit/linuxkit,furious-luke/linuxkit,JohnnyLeone/linuxkit,ndauten/linuxkit,zx2c4/linuxkit,yankcrime/linuxkit,thebsdbox/linuxkit,djs55/linuxkit,justincormack/linuxkit,linuxkit/linuxkit,thebsdbox/linuxkit,JohnnyLeone/linuxkit,JohnnyLeone/linuxkit,ndauten/linuxkit,ndauten/linuxkit,justincormack/linuxkit,YuPengZTE/linuxkit,yankcrime/linuxkit,deitch/linuxkit,deitch/linuxkit,YuPengZTE/linuxkit,thebsdbox/linuxkit,yankcrime/linuxkit,YuPengZTE/linuxkit,thebsdbox/linuxkit,yankcrime/linuxkit,deitch/linuxkit,zx2c4/linuxkit,justincormack/linuxkit,konstruktoid/linuxkit,konstruktoid/linuxkit,yankcrime/linuxkit,linuxkit/linuxkit,djs55/linuxkit