configure: rename QEMU_GA_MSI_ENABLED to CONFIG_QGA_MSI

configure: rename QEMU_GA_MSI_ENABLED to CONFIG_QGA_MSI

The QEMU_GA_MSI_ENABLED config-host.mak variable is emitted by
./configure. meson.build actually checks for CONFIG_QGA_MSI_ENABLED:

  summary_info += {'QGA MSI support':   config_host.has_key('CONFIG_QGA_MSI_ENABLED')}

Rename QEMU_GA_MSI_ENABLED to CONFIG_QGA_MSI for consistency with
CONFIG_QGA_VSS. Also use 'y' instead of 'yes' for consistency.

This fixes the feature summary printed by meson.build.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20200914095231.621068-4-stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>