meson: Add optional dependency on IGVM library

meson: Add optional dependency on IGVM library

The IGVM library allows Independent Guest Virtual Machine files to be
parsed and processed. IGVM files are used to configure guest memory
layout, initial processor state and other configuration pertaining to
secure virtual machines.

This adds the --enable-igvm configure option, enabled by default, which
attempts to locate and link against the IGVM library via pkgconfig and
sets CONFIG_IGVM if found.

The library is added to the system_ss target in backends/meson.build
where the IGVM parsing will be performed by the ConfidentialGuestSupport
object.

Signed-off-by: Roy Hopkins <roy.hopkins@randomman.co.uk>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Gerd Hoffman <kraxel@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Ani Sinha <anisinha@redhat.com>
Link: https://lore.kernel.org/r/45945a83a638c3f08e68c025f378e7b7f4f6d593.1751554099.git.roy.hopkins@randomman.co.uk
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>