monitor: generalize query-mshv/"info mshv" to query-accelerators/"info accelerators"

monitor: generalize query-mshv/"info mshv" to query-accelerators/"info accelerators"

The recently-introduced query-mshv command is a duplicate of query-kvm,
and neither provides a full view of which accelerators are supported
by a particular binary of QEMU and which is in use.

KVM was the first accelerator added to QEMU, predating QOM and TYPE_ACCEL,
so it got a pass.  But now, instead of adding a badly designed copy, solve
the problem completely for all accelerators with a command that provides
the whole picture:

    >> {"execute": "query-accelerators"}
    << {"return": {"enabled": "tcg", "present": ["kvm", "mshv", "qtest", "tcg", "xen"]}}

Cc: Praveen K Paladugu <prapal@microsoft.com>
Cc: Magnus Kulke <magnuskulke@linux.microsoft.com>
Suggested-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>